To provide a combustion efficiency improver for a combustion engine which includes a superior effect of improving combustion efficiency.

The combustion efficiency improver for the combustion engine is produced by combining (i) a fuel modifier for a combustion engine which is composed of a ceramic molding containing a radioactive material to be used while contacting with the fuel in the fuel tank of the combustion engine, and (ii) a gas modifier for the combustion engine which is composed of a heat-resistant resin sheet containing a radioactive material to be used while attached in a place in which it contacts with the air on a line on which the combustion air for the engine passes.