To provide the accumulator built-in injector of an accumulator fuel injection system, by which hydraulic force in a valve closing direction can be applied to a piston when a nozzle needle is closed without deteriorating both points that the size of an injector main body can be diminished in a diametral direction and leak fuel is few.

A large diametral part 42 by which a control chamber formed in the inside of a split member 23 fitted in the rear end part of a nozzle holder 22 composing an injector main body 12 is partitioned into an upper control chamber 45 and a lower control chamber 46, is formed on the head part of a piston which vertically slides in the inside of the nozzle holder 22 integrated with a nozzle needle 11, and also an axial hole 47 and an orifice 48 by which the upper control chamber 45 and the lower control chamber 46 are communicated with each other are formed in the large diametral part 42. Hereby, when a two-way solenoid valve 9 is closed, time lag is produced until hydraulic force in a nozzle opening direction and hydraulic force in a nozzle closing direction which are applied to the large diametral part 42 of the piston 13 are balanced with each other.