To provide a low-cost hollow encoder for a hollow motor that can drastically reduce industrial waste on manufacturing by effectively utilizing the empty space of a mechanism being equipped with the hollow motor.

In the encoder, the rotary angle of a hollow motor 1 where the center of a rotary shaft is in a hollow shape is detected. In the hollow encoder 2 with the same shape as the above hollow shape of the hollow motor 1, a processing circuit board 3 for processing the detection signal of the above rotary angle is mounted to a position where a hollow hole 5 of the above rotary shaft cannot be blocked. The processing circuit board 3 is in a non-donut shape and is arranged so that it jumps out in a projection shape from the outer- periphery surface of the rotary shaft. The processing circuit board 3 is arranged in the same direction as the mounting position of a connector terminal part 4 of the above hollow-type motor when viewed from the center line of the above rotary shaft.