A driving mechanism for driving a gear pump of a dual drive type having two shafts comprises a reversing gear having a pair of output shafts which rotate respectively in opposite directions at the same rotating speed, and a pair of gear couplings connecting the gear pump to the reversing gear. One of the pair of gear couplings comprises: a shaft, a first coupling head disposed on one end of the shaft, and a second coupling head disposed on the other end of the shaft. The number z.sub.1 of teeth of the component external spur gears and internal spur gears of the first coupling head is not equal to the number z.sub.2 of teeth of the component external spur gears and internal spur gears of the second coupling head. The angular difference between the external spur gears of the second coupling head is converted into a corresponding number of teeth of the external spur gear of the third coupling sleeve by calculation using the difference between the central angle including one tooth of the external gear of the first coupling head and the central angle including one tooth of the external gear of the second coupling head, and the third coupling sleeve is turned through an angle corresponding to the calculated number of teeth.