A valve opening/closing timing control device capable of quickly determining attainment of an intermediate locked phase includes: an intermediate lock mechanism capable of switching between a locked state in which a lock member provided in a drive-side rotary member protrudes and fits into a recess provided in a driven-side rotary member, thereby constraining a relative rotation phase to an intermediate locked phase, and an unlocked state in which the lock member withdraws from the recess, thereby releasing the constraint; a phase control unit that controls supply of a fluid to a retard chamber and discharge of the fluid from an advance chamber, or controls discharge of the fluid from the retard chamber and supply of the fluid to the advance chamber, such that the lock member attains the intermediate locked phase; and a determination unit that determines whether the lock member will attain the determination phase, when control has been performed such that the lock member moves toward a determination phase that has been set at a different position than the intermediate locked phase in the recess, after execution of control to supply the fluid to the retard chamber and discharge the fluid from the advance chamber, or control to discharge the fluid from the retard chamber and supply the fluid to the advance chamber. When the result of that determination is that the lock member will not attain the determination phase, the determination unit determines that the relative rotation phase is in the locked state.