A light scanning type object detecting device includes a mirror unit in which first and second mirror surfaces are formed so as to incline in respective directions intersecting with a rotation axis and to face each other with a predetermined angle, a light source; and a light receiving element. On the assumption that H represents a distance between an intersection point of extension lines of lateral sides and a bottom side in the first mirror surface, r represents a radius of a received light flux, h represents a distance between the center of the received light flux and the bottom side, and H′ represents a distance between a top side and the bottom side, formulas (1) and (2) are satisfied. when r