Abstract: Disclosed is a power tool of pen hold type which comprises, within a substantially tubular case having a tip tapered end, a prime mover, a power transmission mechanism connected to the prime mover, a clutch mechanism connected to the power transmission mechanism, a driven section connected to the clutch mechanism when it is made "on", a collet extending from the driven section axially of the case and exteriorly of the tip tapered end thereof, an index mechanism, a switch operating mechanism, and a power input switch, and in which when an operator depresses the switch operating mechanism with his finger while holding the case just as he holds a writing pen with his hand, the switch is closed and simultaneously the index mechanism is brought to a disengaged condition to permit the clutch mechanism to be brought to an engaged condition to permit a transmission of a rotation force from the prime mover to the collet to make the power tool ready for application to operations, and in which when the operator releases