To provide a hydraulic fluid replacement method of a hydraulic cylinder drive circuit for replacing almost all hydraulic fluids in a reciprocating type hydraulic cylinder and a drive circuit by one operation by associated operations of two kinds of circuit configurations formed of a multifunctional valve and a direction selecting valve.

The hydraulic fluid replacement method of the operation circuit of a hydraulic cylinder 10 is performed by successively operating while adjusting an operation amount, a first operation which is the operation of a first circuit configuration (drive circuit) formed of the multifunctional valve and the left selecting position 35c of the direction selecting valve 35, a second operation which is the operation of the second circuit configuration (flushing circuit) of the multifunctional valve and the right selecting position 35b of the direction selecting valve 35, a third operation which is the operation of the first circuit configuration of the multifunctional valve and the right selecting position of the direction selecting valve 35, and a fourth operation which is the operation of the second circuit configuration formed of the multifunctional valve and the left selecting position.