Parallax Scroll Animationの作り方｜スクロールで奥行きを出すUIをJavaScriptで実装

<section class="parallax-hero">
  <div class="parallax-bg layer" data-speed="0.15"></div>
  <div class="parallax-orb orb-1 layer" data-speed="0.35"></div>
  <div class="parallax-orb orb-2 layer" data-speed="0.6"></div>
  <div class="parallax-orb orb-3 layer" data-speed="0.25"></div>

  <div class="hero-content layer" data-speed="0.2">
    <p class="eyebrow">Scroll Motion Pattern</p>
    <h1>Parallax Scroll Animation</h1>
    <p class="lead">
      Different layers move at different speeds to create depth.
    </p>
  </div>
</section>

<section class="content">
  <div class="content-inner">
    <h2>Why it feels good</h2>
    <p>
      Parallax creates a subtle sense of depth with only a small amount of motion.
      It works especially well in hero sections, landing pages, and visual storytelling.
    </p>

    <h2>Implementation</h2>
    <p>
      In this demo, each layer reads the current scroll position and moves vertically
      based on its own speed factor.
    </p>

    <h2>Tips</h2>
    <p>
      Keep the effect subtle. If everything moves too much, readability drops and the page
      can feel unstable.
    </p>

    <div class="spacer"></div>
  </div>
</section>

* {
  box-sizing: border-box;
}

html,
body {
  margin: 0;
  padding: 0;
}

body {
  font-family: Arial, sans-serif;
  color: #e8ecf1;
  background: #0b1020;
  line-height: 1.7;
}

.parallax-hero {
  position: relative;
  min-height: 100vh;
  overflow: hidden;
  display: grid;
  place-items: center;
  padding: 24px;
  background:
    radial-gradient(circle at 20% 20%, rgba(90, 140, 255, 0.28), transparent 30%),
    radial-gradient(circle at 80% 30%, rgba(120, 255, 210, 0.18), transparent 28%),
    linear-gradient(180deg, #10172d 0%, #0b1020 100%);
}

.parallax-bg {
  position: absolute;
  inset: -10%;
  background:
    radial-gradient(circle at 30% 40%, rgba(255, 255, 255, 0.08), transparent 18%),
    radial-gradient(circle at 70% 60%, rgba(255, 255, 255, 0.06), transparent 22%);
  pointer-events: none;
}

.parallax-orb {
  position: absolute;
  border-radius: 999px;
  filter: blur(4px);
  opacity: 0.9;
  pointer-events: none;
}

.orb-1 {
  width: 220px;
  height: 220px;
  top: 14%;
  left: 10%;
  background: linear-gradient(135deg, rgba(123, 92, 255, 0.9), rgba(78, 205, 255, 0.55));
}

.orb-2 {
  width: 300px;
  height: 300px;
  right: 8%;
  top: 20%;
  background: linear-gradient(135deg, rgba(58, 209, 154, 0.8), rgba(115, 224, 255, 0.35));
}

.orb-3 {
  width: 180px;
  height: 180px;
  left: 22%;
  bottom: 10%;
  background: linear-gradient(135deg, rgba(255, 120, 180, 0.7), rgba(255, 210, 120, 0.4));
}

.hero-content {
  position: relative;
  z-index: 2;
  width: min(800px, 100%);
  text-align: center;
  padding: 40px 24px;
  border: 1px solid rgba(255, 255, 255, 0.08);
  background: rgba(255, 255, 255, 0.05);
  backdrop-filter: blur(10px);
  border-radius: 24px;
  box-shadow: 0 20px 60px rgba(0, 0, 0, 0.28);
}

.eyebrow {
  margin: 0 0 12px;
  font-size: 13px;
  letter-spacing: 0.12em;
  text-transform: uppercase;
  color: #8fb4ff;
}

.hero-content h1 {
  margin: 0;
  font-size: clamp(36px, 7vw, 72px);
  line-height: 1.05;
}

.lead {
  margin: 18px auto 0;
  max-width: 560px;
  font-size: 18px;
  color: #c8d4e3;
}

.content {
  background: #f6f8fb;
  color: #1d2433;
}

.content-inner {
  width: min(820px, calc(100% - 40px));
  margin: 0 auto;
  padding: 80px 0 120px;
}

.content h2 {
  margin-top: 0;
  margin-bottom: 12px;
  font-size: 28px;
}

.content p {
  margin-top: 0;
  margin-bottom: 48px;
  color: #4b5568;
}

.spacer {
  height: 60vh;
}

@media (prefers-reduced-motion: reduce) {
  .layer {
    transform: none !important;
  }
}

<script>
  const layers = document.querySelectorAll(".layer");

  const updateParallax = () => {
    const scrollY = window.scrollY;

    layers.forEach((layer) => {
      const speed = parseFloat(layer.dataset.speed || 0);
      const y = scrollY * speed;
      layer.style.transform = `translateY(${y}px)`;
    });
  };

  updateParallax();
  window.addEventListener("scroll", updateParallax, { passive: true });
</script>

const y = scrollY * speed;
layer.style.transform = `translateY(${y}px)`;

<div class="parallax-orb orb-1 layer" data-speed="0.35"></div>
<div class="parallax-orb orb-2 layer" data-speed="0.6"></div>
<div class="parallax-orb orb-3 layer" data-speed="0.25"></div>