To provide a fuel injection type 4-cycle engine which can correct the error between the detection value of a throttle opening sensor and a real throttle opening simply.

In a fuel injection type 4-cycle engine 9, an air is introduced in a cylinder 11 through a throttle valve 72 and a fuel from an injector 76 is jetted and mixed to this air. A control device 111 decides the fuel injection amount of the injector based on the opening of a throttle valve and compares the detection value S of a throttle opening sensor 74 with advancely set idle set opening D and decides a throttle opening correction value H and remembers when a correction value decision signal is inputted. Thereafter, the opening of the throttle valve is found out by correcting the detection value detected by the throttle opening sensor by a memorized throttle opening correction value.