We have proposed a new and innovative gas turbine system, named chemical gas turbine system. It would improve the thermal efficiency more than 10% compared to conventional ones. This gas turbine system is based on promising developments in fuel-rich combustors with a carbon fiber reinforced carbon composite (C/C composite) being used as the turbine blades. As one of fundamental researches to develop this system, we designed a 4 MPa-scale combustor with methane-air. Flammability limit, components of combustion gases and combustion temperature were measured between 1.1 and 4.1 MPa in pressure. Results from these measurements were as follows: (1) stable combustion could attain between the equivalence ratio phi = 0.7 and 1.3 at 4 MPa in pressure; (2) there was little effect of the pressure on the components of combustion gases; and (3) flammability limit extended with increasing the pressure in the fuel-rich region while it was almost constant in the fuel-lean one.