A controller of an internal combustion engine includes: a purge controller, operable to control a purge amount of the vaporized fuel purged to an air intake system; an exhaust air-fuel ratio detector; an air-fuel ratio feedback controller; a period determiner, operable to determine whether it is an estimation enabling period; and an alcohol concentration estimator, operable to estimate the alcohol concentration. When the purge of the vaporized fuel is cut during the estimation enabling period, the purge controller: performs control so that the purge is cut with a large degree of tailing, in an operation region where a load of the internal combustion engine is high and a rotation speed is high; and performs control so that the purge is cut with a smaller degree of tailing than the large degree of tailing, in an operation region where the load of the internal combustion engine is low and the rotation speed is low.