PURPOSE: To improve an exhaust characteristic and operability by changing the time constant of intake pipe volumetric effect according to each of different valve lift characteristics, thereby flattening an air-fuel ratio at the transient time even in an engine provided with a variable valve system.

CONSTITUTION: According to the operating condition determined by a variable valve system 62 and a sensor 63 for detecting engine load, a following cam position is selected by a means 64 and compared with the present cam position by a sensor 65 so as to judge whether to perform cam change-over or not by a means 66, and a change-over signal is outputted to the variable valve system 62. On the other hand, a means 70 selects the appropriate volumetric efficiency from a volumetric efficiency storage means 68 stored with the volumetric efficiency corresponding to each cam, and a time constant calculating means 71 calculates a time constant. The basic injection quantity proportional to the cylinder air inflow quantity is thus computed to flatten an air-fuel ratio at the transient time and thereby to improve an exhaust characteristic and operability.