An advancing check valve is placed in an advancing connection passage of a spool to enable a flow of hydraulic fluid in a first direction from a retarding port side toward an advancing port side upon placement of the spool in an advancing position and to limit a flow of hydraulic fluid in a second direction from the advancing port side toward the retarding port side upon placement of the spool in the advancing position. A retarding check valve is placed in a retarding connection passage of the spool to enable a flow of hydraulic fluid in the second direction upon placement of the spool in a retarding position and to limit a flow of hydraulic fluid in the first direction upon placement of the spool in the retarding position.