Abstract: In an inner-cam system fuel injection pump, two balance ports that are offset symmetrically relative to a distribution port in a circumferential direction to form a Y-shape are formed at a rotor. The opening ends of the balance ports and the opening end of the distribution port are positioned on the same plane that is perpendicular to the axis of the rotor. The offset angle .theta. by which the balance ports are offset in the circumferential direction relative to the axis of the distribution port, the opening area S.sub.1 of the distribution port and the opening area S.sub.2 of each of the balance ports are set to satisfy a relationship expressed as S.sub.1 =.vertline.2.multidot.S.sub.2 .multidot.cos .theta..vertline.. The phase intervals of the fuel delivery passages and the phase interval of the two balance ports are set equal to each other.