To machine an involute curved part of a scroll-like wall in a work piece regardless of existence/absence of eccentricity between the center of rotation of the work piece and the center of the involute base circle.

Circular interpolation is performed for the wall surface shape of the involute curved part 35. A scroll-like work piece is rotated so that the normal direction of the wall surface at a contact point of the wall surface and a tool 16 is always pointed in a fixed direction of X-axis, and the tool is relatively moved in the X-axis direction to the work piece and in the Y-axis direction along the circular arc.