A rotational force transmission mechanism includes a shaft and an engagement member. The shaft is rotatable about a first axis. The shaft includes a leading end portion having a second groove and a first groove which is configured to receive a rotatable portion of a tool holder so that the shaft rotates the rotatable portion about the first axis. The engagement member is movable along the first axis between a first position and a second position and rotatable about the first axis together with the shaft. The engagement member includes an engagement portion configured to be received in the second groove. At least one of the shaft and the engagement member has an inclined surface that moves the engagement portion in a direction toward the first axis when the engagement member moves from the first position to the second position.