To provide a serial parallel switching double gear pump and a switching circuit as a fluid supplying system wherein a temperature rise of fluid is restrained, even when the consumption of fluid such as fuel is decreased in contrast to the number of rotations of a drive system such as a jet engine, and variable displacement, and reduced size and weight are realized with a simple structure.

The serial parallel switching gear pump is equipped with a fuel pump 2 which is a triple gear pump having a first booster 9 and a second booster 10, a switching hydraulic circuit 35 switchable to/from a normal mode for discharging a predetermined amount of fluid with the first booster 9 and the second booster 10 connected in parallel with a variable throttle 34 in a fully opened state, and a half mode for discharging substantially half the amount of fluid of the normal mode with the first and second boosters 9 and 10 connected in series by making the middle pressure between the first booster 9 and the second booster 10 equal to the discharge pressure of the second booster 10, and by putting the second booster 10 in a no load state with the variable throttle 34 in a fully opened state.