An intake air mass estimation unit is provided that sets predetermined degrees of crank angle to an angle that can divide an intake stroke into a plurality of sections, measures at every the predetermined degrees of crank angle the pressure downstream of the throttle valve and the time taken for the predetermined degrees of crank angle rotation, estimates the intake air mass flowing from the upstream to downstream of the throttle valve at every the predetermined degrees of crank angle, using the pressure downstream of the throttle valve and the time taken for the predetermined degrees of crank angle rotation measured at every the predetermined degrees of crank angle, and integrates the intake air mass at every the predetermined degrees of crank angle for 720 degrees of crank angle rotation, thereby estimating the intake air mass needed for one combustion.