To provide a measuring method and a device for an inner and outer diameter of a three-point type circular workpiece, being supplied by simple constitution and inexpensively and moreover having durability to perform stable and accurate inner and outer diameter measuring.

This device has a means for coaxially moving measuring piece fitting blocks 13 and 15 equipped with two measuring pieces 11 and 12 and one measuring piece 14 respectively, a means for stopping or fixing the block 13 at a reference position, means 6 and 7 for moving the pieces 11, 12, and 14 so as to abut on the inner or outer peripheral surfaces of a circular workpiece 20, a means for detecting the moving distance of the piece 14, a means for calculating a correction value about the enlargement ratios of the inner and outer diameters of the workpiece from the moving distance of the piece 14, and a means for calculating the inner and outer diameters of the workpiece 20 by multiplying the correction value to the actually measured value of the workpiece 20.