A valve opening/closing timing control device, comprising: a drive-side rotary member that rotates synchronously with a crankshaft of an internal combustion engine; a driven-side rotary member that rotates in unity with a camshaft of the internal combustion engine, and is capable of relative rotation with respect to the drive-side rotary member; a fluid pressure chamber that is formed by the drive-side rotary member and the driven-side rotary member; a vane that is disposed within the fluid pressure chamber, and divides the fluid pressure chamber into a retard chamber and an advance chamber that permit inflow or discharge of a fluid, and selectively moves a relative rotation phase of the driven-side rotary member with respect to the drive-side rotary member between a retard direction in which volume within the retard chamber increases due to inflow of the fluid and an advance direction in which volume within the advance chamber increases due to inflow of the fluid; an intermediate lock mechanism that includes a lock member provided in any one of the drive-side rotary member and the driven-side rotary member and is capable of moving with respect to the other, and a recess that extends in a circumferential direction and is provided in the other of the drive-side rotary member and the driven-side rotary member, the intermediate lock mechanism being capable of switching between a locked state in which the relative rotation phase is constrained to an intermediate locked phase between the most advanced phase and the most retarded phase by the lock member fitting into the recess, and an unlocked state in which the constraint has been released by the lock member withdrawing from the recess; a phase control unit that controls supply of the fluid to the retard chamber and discharge of the fluid from the 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 characterised by a determination unit that, after the phase control unit executed the control such that the lock member attained the intermediate locked phase, causes the phase control unit to perform control to move the lock member toward a determination phase that has been set at a different position than the intermediate locked phase in the recess, and determines whether the lock member has attained the determination phase, and when the result of that determination is that the lock member has not attained the determination phase, the determination unit determines that the lock member is in the locked state. a drive-side rotary member that rotates synchronously with a crankshaft of an internal combustion engine; a driven-side rotary member that rotates in unity with a camshaft of the internal combustion engine, and is capable of relative rotation with respect to the drive-side rotary member; a fluid pressure chamber that is formed by the drive-side rotary member and the driven-side rotary member; a vane that is disposed within the fluid pressure chamber, and divides the fluid pressure chamber into a retard chamber and an advance chamber that permit inflow or discharge of a fluid, and selectively moves a relative rotation phase of the driven-side rotary member with respect to the drive-side rotary member between a retard direction in which volume within the retard chamber increases due to inflow of the fluid and an advance direction in which volume within the advance chamber increases due to inflow of the fluid; an intermediate lock mechanism that includes a lock member provided in any one of the drive-side rotary member and the driven-side rotary member and is capable of moving with respect to the other, and a recess that extends in a circumferential direction and is provided in the other of the drive-side rotary member and the driven-side rotary member, the intermediate lock mechanism being capable of switching between a locked state in which the relative rotation phase is constrained to an intermediate locked phase between the most advanced phase and the most retarded phase by the lock member fitting into the recess, and an unlocked state in which the constraint has been released by the lock member withdrawing from the recess; a phase control unit that controls supply of the fluid to the retard chamber and discharge of the fluid from the 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 characterised by a determination unit that, after the phase control unit executed the control such that the lock member attained the intermediate locked phase, causes the phase control unit to perform control to move the lock member toward a determination phase that has been set at a different position than the intermediate locked phase in the recess, and determines whether the lock member has attained the determination phase, and when the result of that determination is that the lock member has not attained the determination phase, the determination unit determines that the lock member is in the locked state.