To provide an apparatus which enables a cutting along a generating line which agrees with the thickness direction of a board to be cut, with a desired arc length maintained.

The first holding part 2 which can move vertically in the middle of an apparatus, the second and third holding parts 3, 4 which are installed opposite to each other on both sides of the first holding part and can change the distance from the first holding part and rotate, circular saws installed in the second and third holding parts while moving along a generating line, and a control part B for controlling the arrangement of the second and third holding parts are provided.