PURPOSE: To apply a forward oriented force to a tap and prevent 'thread thinning' due to generation of any hindrance from normal advancing by furnishing a tap holding shaft with a contacting part in such an arrangement as to contact the rear face of a piston in the condition that the holding shaft lies in its regular position relatrive to the body of a tap holder tool.

CONSTITUTION: A pressure chamber 40 in communication to a machining oil path 25 is provided in a tap holding shaft 21 which is capable of advancing and retreating relative to the body 5 of a tap holder tool. A piston 41 hindered from retreating by the body is installed in the pressure chamber 40. When a tap 23 is hindered from normal advancing due to a pressure rise of the machining oil in a lower hole 61, a contacting part 44 provided on the holding shaft 21 separates from the rear face of the piston 41, which causes application of a forward oriented force to the holding shaft 21. This force is applied to the tap 23 so that it is advanced.