To provide a focal position measurement device and a method for a camera interchangeable lens facilitating the focal position measurement, and enhancing the accuracy.

This focal position measurement device 1 for the camera interchangeable lens is composed of a slit plate 10 having a slit which is irradiated with outgoing light from a light source 7, a collimator lens 12 having chromatic aberration to convert the light leaving the slit plate 10 to a parallel luminous flux and cause the parallel luminous flux to enters a test lens 3 to be collected, and a mirror 4 arranged in vicinity to the focal position of the test lens 3 to be moved to-and-fro of the focal position along the optical axis and reflect the light collected through the test lens 3. The light reflected by the mirror 4 is caused to enter the test lens 3 to be converted to a parallel luminous flux again. The luminous flux is collected through the collimator lens 12 to form a slit image. The focal position of the test lens 3 is determined based on a chromatic change in the slit image corresponding to the movement of the mirror 4.