diff --git a/sphinx/sphinx_to_html.py b/sphinx/sphinx_to_html.py
index 95bcad9..8e0f981 100755
--- a/sphinx/sphinx_to_html.py
+++ b/sphinx/sphinx_to_html.py
@@ -95,7 +95,8 @@ with tempfile.TemporaryDirectory() as tmp_dir:
     out_dir = work_dir / "out"
     out_filepath = out_dir / "contents.html"
 
-    sphinx_cmd = ["sphinx-build", "-b", "html", work_dir, out_dir]
+    sphinx_build = script_dir / "venv" / "bin" / "sphinx-build"
+    sphinx_cmd = [sphinx_build, "-b", "html", work_dir, out_dir]
     result = subprocess.run(sphinx_cmd, capture_output=True)
 
     # Output errors.