A hydraulic control circuit adequate to control a main hydraulic actuator and auxiliary hydraulic actuators. The main actuator drives farm machinery mounted to an agricultural tractor or the like. The auxiliary actuators drive farm attachments independent of said farm machinery. At least two hydraulic pumps and at least two sets of control valves are connected in series. The exit of one hydraulic pump is connected with the other hydraulic pump via a check valve. Said one hydraulic pump is connected with a tank via an openable valve upstream of the check valve. The upstream set of control valves is used to control the auxiliary hydraulic actuators needing more flow rate than the downstream set of control valves. The openable valve has an offset valve at its one end to determine the direction of flow. The openable valve is either opened or closed by the pressure admitted from the downstream side of the upstream set of control valves. Thus, when the upstream set of control valves needing more flow rate is not operated, only the oil delivered from the other hydraulic pump flows into the circuit. When the upstream set of valves is operated, the oil delivered from one pump meets the oil delivered from the other pump, increasing the pump capacity. Hence, the speed of the farm attachments can be adjusted without the need to modify the preset speed of the machinery.