To provide a gas fuel internal combustion engine capable of preventing variations of an air-fuel ratio by reducing a passage length of a leak gas introduction passage and improving assembling workability.

A gas fuel internal combustion engine includes a leak gas introduction passage 27 for introducing to a scavenging port 3c gas fuel entering between a valve guide member 24 and valve stem 20a. The leak gas introduction passage 27 which extends in a cylinder head 4 toward a mating surface 44 between the cylinder block 3 and the cylinder head 4 at the scavenging port 3c side from a gap with the valve stem 20a of the valve guide member 24 is formed in the internal combustion engine 1 so as to pass the cylinder block 3 from the mating surface 44.