Screw tightening tool

A screw tightening tool comprising:

• a case body including a first wall portion, a second wall portion, and a guide portion, the first wall portion and the second wall portion being opposed to each other with a specified gap, the guide portion being configured such that a head portion of a screw is inserted therein, the guide portion having a substantially cylindrical shape and protruding to a first direction from the specified gap, the first direction being orthogonal to an opposition direction, the opposition direction being a direction in which the first wall portion and the second wall portion are opposed to each other;
• a tool member including a base portion and an engaging portion, the base portion being provided between the first wall portion and the second wall portion, the base portion having a substantially circular shape and having an axis line that is parallel with the opposition direction, the engaging portion protruding from the base portion to a direction that is orthogonal to the opposition direction, a bit portion that is configured to engage with the head portion being provided on a leading end of the engaging portion;
• and
• a locking mechanism including a groove portion and a case body locking portion, the groove portion being provided in a surface portion of the base portion, the surface portion being opposed to the first wall portion, the groove portion extending in a substantially circular arc around the axis line, the case body locking portion protruding from the first wall portion toward the second wall portion, a leading end of the case body locking portion being arranged inside the groove portion, wherein:
• the case body rotatably supports, between the first wall portion and the second wall portion, the base portion such that the base portion rotates around the axis line,
• a rotation position of the tool member switches at least among a first position, a second position, and a third position in accordance with a rotation of the base portion, the first position being a rotation position in which the bit portion protrudes to an outside of the case body, the second position being a rotation position in which the bit portion protrudes to the outside of the case body and that is different from the first position, the third position being a rotation position in which the case body houses the bit portion, and the bit portion being configured to, in the third position, engage with the head portion inserted in the guide portion,
• the leading end of the case body locking portion slides relative to the base portion inside the groove portion in accordance with the rotation of the base portion, and
• the locking mechanism includes a first lock portion, a second lock portion, and a third lock portion, the first lock portion, the second lock portion, and the third lock portion being provided in the groove portion, the first lock portion locking the leading end of the case body locking portion when the tool member is in the first position, the second lock portion locking the leading end of the case body locking portion when the tool member is in the second position, the third lock portion locking the leading end of the case body locking portion when the tool member is in the third position.