PURPOSE: To provide an inner cam distribution type fuel injection pump which prevents the fuel leak from a cam chamber.

CONSTITUTION: As the engine is started, the fuel supplied from a vane feed pump 1 to a feed gallery 5 is introduced into a plunger chamber 9 in the suction stroke the fuel in the plunger chamber 9 is highly pressurized in the compression and distribution stroke and introduced into the distribution passage 23 from the distribution port 16 and the distribution passage 18, and injected from the nozzle through a delivery valve 24. Then, the highly pressurized fuel in the plunger chamber 9 is overflowed by opening an electromagnetic overflow valve 19, and the injection is stopped. The operation of this injection pump 10 allows the pressure of the fuel in the part surrounded by an inner cam ring 14 and a distribution head 4 to be raised, and the fuel is pushed out from the aperture between the two. The highly pressurized fuel pushed out to the outer circumference side of the inner cam ring 14 is expanded after reaching a relief groove 36 to the low pressure. Thus, no highly pressurized fuel is leaked from the cam chamber.