A boring method includes: preparing a plurality of binding members (64) which differ from one another in diameter of an orifice (641); mounting the plurality of the binding members (64) in a branch flow path (63) by sequentially replacing the binding members (64); discharging an insert plug from a main flow path end which blocks the main flow path and measuring a pressure of a fluid before passing through the orifice by mounting a pressure gage on the main flow path, and discharging the insert plug of the flow path which blocking the flow path from a bushing face to exterior formed in support jigs and mounting the pressure gage to measure the pressure of the fluid after passing through the orifice; selecting one of the plurality of the binding members (64), which has a ratio between the pressure of the fluid before passing through the orifice and the pressure of the fluid after passing through the orifice, which is closest to a predetermined ratio value, and mounting the selected binding member in the branch flow path; and supplying the fluid to the main flow path (61) of a cutting tool (30), thereby rotating the cutting tool (30) so as to bore the workpiece (W).