To provide a processing method and a device for improving productivity by reducing cutting time in processing a groove hole of a square cylindrical work.

This processing device 1 comprises a work support part 5 and a cutter support part 10 arranged on a machine table 3. The work support part 5 is movably constituted in the Y axis direction by positioning and fixing the work W. The cutter support part 10 is installed so that a circular saw blade 7 inclines to a rotary shaft of an installation attachment 40, and is movably constituted in the X axis direction and the Z axis direction. The groove hole is formed in the work W with a dimension between an upper end position and a lower end position of the circular saw blade 7 as a groove width dimension by slantingly installing the circular saw blade 7 to the axis.