To obtain a method and an apparatus in which the rotational position in both directions at a specific angle or higher of a rotating body can be detected, by a method wherein a watch dog for sensor which is installed at an endless belt body is detected by a sensor which is arranged between the respective rotating bodies.

A first rotating body 1 is turned to a direction CW, and a watch dog 4 for a sensor is moved to a position indicated by a dotted line together with an endless belt body 2. Then, the position is detected by a sensor 5, and, in this state, the rotating body 1 is situated on a position which is turned by about 360° to the direction CW. In addition, the rotating body 1 is turned to a direction CCW, and another dog 4 for a sensor is moved to a position indicated by a dotted line together with the endless belt body 2. Then, the position is detected by a sensor 5A, and, in this state, the rotting body 1 is situated in a position which is turned by about 360° to the direction CCW. In addition, the angle of rotation or the speed of rotation of the rotating body 1 can be set arbitrarily on the basis of the length of the intershaft distance L between the rotating body 1 and a rotating body 3 and on the basis of arrangement and installation poisons of the sensors 5, 5A. Consequently, the rotational position in both directions at 180° or larger of the rotating body 1 can be detected.