Provided is a threaded member tightening tool capable of returning a count value in linkage with reverse driving by the threaded member tightening tool and only when a threaded member to be removed by the reverse driving has been counted as a tightened threaded member. A computing unit of a motor-driven screwdriver is switchable between a counting mode in which the computing unit counts the number of tightened threaded members and holds the counted number as a count value and a count return mode for returning the count value to a one-preceding count value. The computing unit returns the count value to a one-preceding count value based on the fact that the engaging element fitting unit has been driven in the reverse direction when the computing unit has been switched to the count return mode.