Control device of internal combustion engine

A control device of an internal combustion engine designed to find a response time constant of a cylinder air filling amount or its equivalent value by a simpler method. The control device comprises means for setting the target opening degree, means for finding a target intake pipe pressure, means for finding a current intake pipe pressure, means for finding a throttle valve air passage flow rate, and means for finding a cylinder intake air flow rate and calculating the response time constant based on the target intake pipe pressure, the current intake pipe pressure, the throttle valve air passage flow rate, and the cylinder intake air flow rate.