To provide a gear pump or motor which solves the problem that the use of a gear with a helical tooth form prevents vibration or noise from being generated but makes the generation of thrust force require to hold the gear firmly, particularly, to strengthen a bearing mechanism so that there occur economical disadvantages.

A drive gear 4A and a driven gear 5B of the gear pump or motor GB are composed of double helical gears. As for the drive gear 4B, two helical gears are joined to each other so as to constitute the drive gear 4B. The inclinations of tooth forms of the respective helical gears are reverse to each other, and the inclination angles are the same as each other. Therefore, the double helical gear is constituted by joining the two helical gears having the tooth forms the directions of inclination of which are reverse to each other. A helical gear designed and manufactured to have the same tooth form as the drive gear 4B, that is, the same inclination angle as the drive gear 4B, is employed for the driven gear 5B.