To provide an intake air quantity estimating device for an internal combustion engine capable of exactly estimating intake air quantity by considering the state of residual gas within a combustion chamber at the time of a transition and to provide a fuel supply control device capable of sufficiently suppressing influence on emission and an engine output.

By charging efficiency KTPt (S130) corrected based on an actual combustion chamber temperature Tr and a combustion chamber temperature T at normal time and intake pressure PM, intake air quantity (K1×PM×KTPt) is determined (S150). Therefore, a temperature state of residual gas within a combustion chamber of an engine can be reflected on intake air quantity and exact intake air quantity can be determined also at the time of a transition. Therefore, sufficiently exact fuel injection quantity TAU corresponding to actual intake air quantity can be calculated also at the time of the transition (S150).