PURPOSE: To provide an auxiliary chamfering tool of a board cutter capable of bringing the sharp blade tip of a cutter knife in contact with a cutting surface at all times and freely selecting the cutting direction of a board.

CONSTITUTION: This auxiliary tool main body 11 is formed by two planes folded to an approximately right angle, cut-off parts 21a and 21b are inclined and formed on the folded part of the auxiliary tool main body 11 and chip take-out grooves 22a and 22b are formed respective continuously to the cut-off parts 21a and 21b. Formation is performed so as to form respective boundary parts 23a and 23b with an acute angle to the longitudinal direction of the auxiliary tool main body 11 and to make them be in opposite directions.