PURPOSE: To eliminate the thermal unbalance of air inlet pipe at the time of partial cylinder operation by forming part of an exhaust gas introduction pipe through which the exhaust gas from quiescent gas cylinder groups is also introduced into air inlet pipe integrally with the air inlet pipe of the working gas cylinder groups.

CONSTITUTION: At the lower part of the air inlet pipe 2, the exhaust gas circulating pipe 20 is integrally formed in much the same shape as the air inlet pipe 2 in such a manner that it is isolated from the air inlet path 5. The exhaust gas circulating pipe 20 is provided with the first partition wall 24 by which exhaust gas coming from the inlet 23 is guided toward the upstream side of the air inlet pipe 2, and then the partition wall 24 is provided with the partition walls 26 extending up to near the tipes through the central parts of the paths 25aW25f corresponding to the branched portions of the air inlet path, by which circulating exhaust gas is forcibly guided in order from the lower parts of the branched portions of the first cylinder down to the lower parts of the branched portions of the sixth cylinder and then sent back to the air inlet path 5 through the opening 27.