A liquid pressuring device comprises a reciprocating pump (1), pressure sensor (23) and pressure control unit (25) which is adapted to control the motion of the plungers (5A, 5B) so as to make an actual delivery pressure value reach a predetermined threshold and, after reaching the threshold, control the actual delivery pressure value so as to determine an optimum feed rate, and thereafter maintain the feed rate of the plungers constant at the optimum feed rate. The pressure control means (25) may include a proportional control element for performing, after the actual delivery pressure value has reached the threshold, the proportional control of the actual delivery pressure value during a time until the plungers (5A, 5B) first reach the forward stroke end thereof.