The atmospheric pressure is detected accurately without using an atmospheric pressure sensor. The atmospheric pressure is obtained from an intake air pressure just before an inlet valve opens by detecting an intake air pressure. Since the intake air pressure just before the inlet valve opens becomes unstable when the engine load is large, the atmospheric pressure is calculated from an intake air pressure just before the inlet valve opens when the engine load is small. In addition, since the intake air pressure just before the inlet valve opens depending upon engine rotational speeds, the atmospheric pressure is calculated from an intake air pressure just before the inlet valve opens when the engine rotational speed resides in a predetermined rotational speed area.