A substrate transfer method of delivering a substrate transferred by a transfer arm to a mounting unit and associated apparatus. The method includes moving the transfer arm to a position above the mounting unit, and, when delivering the substrate onto the mounting unit, detecting a mounting portion of the mounting unit by a mounting portion detector having a light emitting element and a light receiving element provided at opposite positions across a center point of the transfer arm on a bottom surface of the transfer arm, and then lowering the transfer arm in an oblique direction to mount the substrate on the mounting unit.