To provide a small-sized and well operable portable screw tightening machine that has a mechanism for inching a screw holding belt in which screws are fitted over a screw fitting interval.

The screw holding belt inching mechanism is fixed on a screw tightening machine body 1. A driver bit 90 is, upon an input operation from an operating means, powered by an electric motor and rotatively driven for forward movement toward objects of connection relatively to the screw tightening machine body 1, and upon an input operation release of the operating means, released from the forward rotational drive and then driven for backward movement. The screw holding belt inching mechanism inches the screw holding belt over one stroke according to a backward movement detecting means for detecting an end state of one forward and backward reciprocating movement of the driver bit 90.