PURPOSE: To perform the measurement readily and highly accurately, by photoelectrically measuring the attaching angles of a plurality of bodies which are attached to a rotary body.

CONSTITUTION: When a disk 1 is rotated at a constant speed and laser light L is outputted from an oscillator 6, the light L is sequentially irradiated on a curved surfaces of a pair of heads 3 and 3 which are rotated together with the disk 1. Scattered light L2 at a slit part 4 of the curved surface is converged by a lens 8, passes through a tramsitting hole 10 in a space filter 9, and is sequentially detected by sensors 11 and 12. At the instant the light moves from the sensor 11 to the sensor 12, a zero crossing point Z1 is obtained. The time of the crossing is stored in a signal processing device 14. Points Z2 and Z3 are stored by the similar way. Then a period T1 from the point Z1 to the point Z2 and a period T2 from the point Z2 to the point Z3 are compared and computed, and the assembling accuracy of the heads 3 and 3 are obtained.