To provide a marking work utensil for bolt head or nut, which is capable of applying and forming a mark ranging in a uniform width through the bolt head, the upper surface of the nut, the side surface, a washer and the upper surface of a tightening member regardless of a revolution position of the bolt head or the nut even if the marking is carried out from a prescribed direction of the tightening member to enable to monitor the mark from a prescribed outside position.

As shown in fig. 1A, a mark head 200A having a cap 220A provided with a slit 230 set in the prescribed direction of the tightening member 2 is covered on the bolt head 31 or the nut 32 and an atomized ink 6 is sprayed toward the slit from the outside position. Only the atomized ink 6 passed through the slit 230 in the sprayed atomized ink 6 continues to be spread in the spray direction, even if a gap is present between the inside diameter part 22011A of a cap main body 2201A and the bolt head 31 or the side surface of the nut, and is stuck to the bolt head 31, the upper surface and side surface of the nut, the washer 4 and the upper surface of the tightening member 2 within a range where the width of the slit 230 is projected to apply and form the mark 5a.