To prevent any fuel leakage at the time of low pressurization so as to practice sufficient fuel pressurization.

The present inner cam type fuel injection pump is provided with a cam ring 1 having cam swells 3 on its inside peripheral surface 2, with a rotor for making rotation in linkage to an engine in radial directionally inside of the cam ring 1 and with a plunger 9 for moving to the inside in radial direction by being pushed by the abovementioned cam swells 3 in the inside of the rotor so as to pressurize and forcibly feed fuel, while the plunger 9 is formed solid in its end part 10 in its radial directional inside and provided with hollow part 12 formed on the radial directional outside from the end part 10.