To provide an evaporated fuel treatment device for an internal combustion engine capable of making purge quantity sufficiently large.

This device is provided with a first fuel condition determination means determining fuel concentration purged from a canister based on slippage quantity of actually measured air fuel ratio from target air fuel ratio, a second fuel condition determination means determining fuel concentration under a condition where a purge control valve is open, an abnormality detection means detecting abnormality of the second fuel condition determination means, and an air fuel ratio control means controlling fuel injection quantity based on fuel concentration. If the abnormality detection means does not detect abnormality, the air fuel ratio control means uses fuel concentration determined by the second fuel condition determination means at a time of purge start or re-start, and uses fuel concentration determined by the first fuel condition determination means during purging. Consequently, purge ratio can be kept large from the time of purge start or re-start. However, fuel concentration determined by the first fuel condition determination means is always used during abnormality diagnosis by the second fuel condition determination means.

COPYRIGHT: (C)2007,JPO&INPIT