QGIS in OSGeo4W (recommended for regular users):

インストーラの中でExpress Install を選択し、 QGIS to install the latest release or QGIS LTR to install the long term release.
The express installations have several optional packages including non-free software. To avoid those you have to use the 上級インストール and choose qgis and/or qgis-ltr in the desktop section.

NOTE FOR EXISTING USERS: OSGeo4W v2 (previously known as testing) is now the regular repository. The latest QGIS release is only available here, as it already requires dependencies not available in the old repository. The long term release is additionally also available in the old repository using the same dependencies as before (see below). This also includes a 32-bit version, which OSGeo4W v2 does not support.
CAUTION: Upgrades of old setups using the new repository are not supported. You need to do a fresh install or use a different directory.
CAUTION: Windows 7 no longer works as we are now using Python 3.9, which dropped support for it.

Standalone installers (MSI) from OSGeo4W packages (recommended for new users)

最新リリース（機能が最も豊富）：

長期リリース（最も安定）：

Note that the MSI installers are much bigger than the previous installers. This is because they include significant larger packages (eg. PROJ 8). The main reason for the switch to MSI were the size limits previously used NSIS has, which was blocking updates of dependencies.

Long-term release in old OSGeo4W (continued with previous dependencies):

インストーラの中で上級インストール を選択し、 qgis-ltr-full to install the long term release.
Packages for the latest release and nightly thereof and master are discontinued in old OSGeo4W.
This installer also allows installing QGIS without non-free software

Standalone installer for long term release (continued with dependencies from old OSGeo4W):