PURPOSE: To provide a cutter for drilling capable of performing a drilling work without needing a high force and preventing entrance of chips to a main pipe during a drilling work.

CONSTITUTION: The blade surface 23 of the tip of the blade part 22 of a cutter 21 for drilling consists of a wave-form sharp edge based on the screw-in direction of the cutter 21 for drilling. Thus, when the cutter 21 for drilling is rotated in a given direction during drilling, the blade part 22 is rotated integrally with a rotation part 4a and cuts through the main pipe 2. In this case, the part protruded to the main pipe 2 side of the waveform blade surface 23 is repeatedly cut through the main pipe 2 at intervals of a specified angular distance. A friction force against a peripheral direction is reduced and drilling is effected as the cutter 21 for drilling is rotated with a comparatively low force. Further, since the blade surface 23 is gently curved, chips are prevented from generation during drilling, and the chips are prevented from entering the main pipe.