PURPOSE: To obtain an absolute type rotary encoder having a high speed response of the same degree as that of an incremental tape while utilizing characteristics of an absolute type.

CONSTITUTION: An absolute signal is obtained by detecting a transmitted light from a circular slit of an absolute part on a rotary disk. The transmitted light from the slit of a trigonometric function on the disk is sensed by a trigonometric function photoreceiver to obtain two types of cosine waves having different phases. The photoreceiver has a cat's eye state on its photoreceiving surface, and outputs cosine waves upon rotation of the disk. The two types of the waves of having different phases are converted into inner insertion split data representing the deviation of a rotation at each one period of the wave. The data is added to the absolute signal to obtain a final absolute value. Thus, an absolute type rotary encoder having a high speed response equal to the incremental type is obtained.