A screw tightening tool includes a case body, a tool member, and a switching portion. The case body includes a guide portion configured such that a head portion of a screw is inserted therein. The tool member includes an engaging portion including a bit portion configured to engage with the head portion. The switching portion supports the tool member. The switching portion is configured to switch a position of the tool member among a first position, a second position, and a third position. The first position is a position in which the bit portion protrudes to an outside from the case body. The second position is a position in which the bit portion protrudes to the outside from the case body and that is different from the first position. The third position is a position in which the case body houses the bit portion.