To provide a manufacturing method and device for a pump which locates the place of eventual failure in the pump without disassembling it and acquiring a presumptive value of the discharging performance for a non- compressive medium from the discharging performance for a compressive medium.

The revolving speed R and the air discharge pressure P of a motor are measured and computed, and from the obtained values, the pump performance index E is determined. Using the revolving speed R of the motor, judgement is made whether the motor part is in acceptable workmanship, and if any failure exists, that effect is displayed, and if the motor part is judged as normal, the pump part is judged whether it is normal using the index E. If any failure exists in the pump part, that effect is displayed. Then a judgement is made for the whole construction of washer pump, using the air discharge pressure P, and if the washer pump is judged as normal, a presumption is made for the water discharging performance from the air discharge pressure P, using the regression equation. If the judgement indicates any failure existing in the washer pump, that effect is displayed.