PURPOSE: To impose the most proper load to an engine at all times by a method wherein an output of a hydraulic pump is controlled in accordance with a relation between an opening degree of a throttle and a number of revolution of the engine in a riding type rice planting machine utilizing H.S.T. (hydrostatic transmission gear).

CONSTITUTION: When a throttle lever 1 is pivoted, a rotary shaft 3 is pivoted and a voltage e1 in accordance with a rotating angle thereof is generated from a potentiometer 2. On the other hand, a voltage e2 in accordance with a number of revolution of an output shaft 4 of an engine is generated from a detecting member 6 in this time. Respective voltages e1, e2 are in a relation of e1=e2 when the most proper load is imposed to the engine, but the load is increased and the relation becomes e2<e1 when a rice planting machine is turned or the like. Then, a contact C1 of a differential type pole relay R is closed and a relay R1 is operated, thereby exciting a left solenoid S1of a switching valve 9. Accordingly, the valve 9 is moved to a position P1 and an inclination angle of a swash plate 14 for a hydraulic pump 13 becomes smaller, thus, a car speed may be reduced.