PURPOSE: To reduce start load while suppling a fluid with predetermined pressure within a short time, in a biochemical automatic analytical apparatus, by applying change-over control to the atomosphere open port of a three-way change-over valve on the basis of the output of a pressure detecting means.

CONSTITUTION: When a power source is connected to a pump 1 to raise air pressure in the vicinity of max. reaching pressure po, a signal is sent to a control circuit 6 from a pressure sensor 5 and a two-way solenoid valve 4 is brought to an open state to exhaust compressed air from a drying nozzle through an exhaust pipe 10. When air pressure reaches a pressure limit value or less for imparting effect to drying, the two-way solenoid valve 4 is brought to a closed state. In the next step, in starting the pump 1, a three-way solenoid valve 2 is changed over to the continuity state with a pipe 7 and a pipe 11 and the output side of the pump 1 is opened to the atmosphere to start the pump 1. After a definite time is elapsed, the three-way solenoid valve 2 is changed over to the continuity state with the pipe 7 and a pipe 8 and the pressure of the pump 1 is transmitted to the succeeding stage to increase the pressure in a flowline to PO.