To reduce the position detection errors of focal position, in a device for measuring the focal position.

The device for measuring the focal position includes a light source part 50 for making a light flux 5b incident on an optical element 1; a mirror 7 that reflects luminous flux 5c which transmitted through the optical element 1 on a reflecting face 7a by possessing the reflecting face 7a that is substantially orthogonal to the optical axis 8a of the optical element 1; a stage 10 for moving the reflecting face 7a in the neighborhood of an image formation position of the luminous flux 5c that transmits the optical element 1; a condensing lens 14 that condenses luminous flux 5d that retransmits the optical element 1, by being reflected on the reflecting face 7a; a light-receiving element 16 for detecting the light amount of a luminous flux 5e condensed on the condensing lens 14; and a pinhole plate 15 that provides an opening 15a, at a position separated by a fixed distance from the condensing lens, on the optical path between the condensing lens 14 and a light-receiving element 16. The device is constituted so that the image-forming position of the luminous flux 5c, that transmits the optical element 1 and the position of the opening 15a of the pinhole plate 15, are conjugated.