This method for detecting a poor module-mounting-state in a concentrator photovoltaic apparatus includes: photographing a surface of an array by an imaging device; obtaining an image in which a virtual image, magnified through a condenser lens, of a light receiving portion including a cell and a vicinity thereof is formed, and a collection of pixels of the virtual image forms a composite virtual image of an entirety of the light receiving portion, the composite virtual image being projected over a plurality of modules; and detecting a poor module-mounting-state based on a form of the composite virtual image.