To make internal diameter measuring accuracy sufficient so as to obtain desired processing dimension, even when a processed hole is formed in an elliptic shape and tapered.

In a honing head 1 provided with a grinding wheel 7, an air nozzle 15 is provided in order to measure an internal diameter of a machining hole H, the honing head 1 is while rotated and moved in an axial direction, a back pressure of ejected air of the air nozzle 15 is converted into an electric signal by an A/E converter 17, a detection data is obtained. A fetching period by a detection data fetching means 21 of this detection data is set shorter than a time in any shorter one of the time required for making 1/4 turn of the honing head 1 and the time required for one moving in an axial direction of the honing head 1.