To limit reaction torque of a drive shaft by a simple structure by providing a target breaking portion on the drive shaft and breaking the drive shaft at the target breaking portion when reaction torque exceeds maximum reaction torque stipulated.

A target breaking portion is formed on a drive shaft 21 by working and forming an annular groove 41 on the peripheral surface of the drive shaft 21. The annular groove 41 is directly provided on the cross section transfer part between a forming head 25 and a shaft part of the drive shaft 21. The depth of the annular groove 41 is locked by a gear 5 which is provided within a pump chamber 3 and is revolved and driven by an external driving element and a gear 9 to which the rotary motion of this gear 5 is transmitted via a peripheral surface gear train. When remarkably large return torque is generated, the drive shaft 21 is made to be broken at this portion. In this case, various kinds and different maximum allowable reaction torque in the drive shaft 21 can be adjusted according to the depth of the annular groove 41.