1. A projection apparatus for projecting a plurality of stereoscopic images such that portions thereof overlap each other and displaying one stereoscopic image, the apparatus comprising: a projection unit adapted to project an indicator that is an image for adjusting a stereoscopic degree corresponding to a parallax between a projected left image and a projected right image, in an overlapping region where the plurality of stereoscopic images overlap each other; a detection unit adapted to detect a parallax between an indicator in the left image and an indicator in the right image based on a captured image of a projected indicator; and an adjustment unit adapted to adjust stereoscopic degrees of the plurality of stereoscopic images by an adjustment amount that is determined based on the detected parallax.