PURPOSE: To provide a nut runner which can surely maintain the engagement of clutch teeth regardless of either of a high speed low torque mode and a low speed high torque mode, and also can be normally/reversely rotated.

CONSTITUTION: The teeth of a top cam 65 and the teeth of a drive shaft which are meshed with each other at the time of a high speed low torque mode, are formed into each rectangular tooth 67 and 68. Both the teeth 67 and the teeth 68 are engaged with each other by the energizing force of a spring 103. The teeth of a bottom clutch 92 and the teeth of an output shaft 36 which are meshed with each other at the time of a low speed high torque mode, are formed into each rectangular tooth 61 and 95. The bottom clutch 92 is held down by air pressure via a piston 76, so that both the teeth 61 and the teeth 95 are thereby engaged with each other.