A device for detecting the positional relationship between first and second objects in a predetermined direction includes a light source for emitting light in a direction to the first or second object, and a first detecting portion for detecting the position of incidence of a first light deflected by the first and second objects, wherein the position of incidence of the first light upon the first detecting portion is changeable with a change in the positional relationship between the first and second objects in the predetermined direction. A second detecting portion detects the position of incidence of a second light deflected by at least one of the first and second objects, wherein the state of the position of incidence of the second light resulting from a change in the positional relationship between the first and second objects, in the predetermined direction differs from that of the first light. On the basis of the detection by the first and second detecting portions, the positional relationship between the first and second objects is detected without being affected by an inclination thereof.