1. An adjustment apparatus for adjusting a stereoscopic degree of at least one of a plurality of projection apparatuses for displaying one stereoscopic image obtained by projecting a plurality of stereoscopic images such that portions thereof overlap each other, the adjustment apparatus comprising: a projection control unit configured to cause a projection unit of at least one of the plurality of projection apparatuses to project a left image including an indicator and a right image including an indicator, wherein the indicators are used to detect a parallax between the left image and the right image, and wherein at least a part of each of the indicators is projected by the projection unit in an overlapping region where the plurality of stereoscopic images overlap each other; a detection unit configured to detect the parallax between the left image and the right image based on a captured image of the indicators which are projected by the projection unit; and an adjustment unit configured to adjust the stereoscopic degree of the projection unit by an adjustment amount that is determined based on a comparison of a parallax between the left image and the right image with a predetermined value.