Steam reforming reaction of methane in direct internal reforming molten carbonate fuel cell was studied. Every experiment was performed in 5 ㎝ by 5 ㎝ unit cell at 650 ℃ and 1 atm, and equilibrium composition calculation was performed simultaneously. The performance of the cell operated with methane showed 140 ㎃/㎠ current density at 0.7 V which differs little from the performance of the cell operated with hydrogen. The stable operating conditions with no risk of carbon deposition on catalysts could be estimated from the equilibrium composition calculation and confirmed by experiments. The risk of carbon deposition could be ruled out in the S/C ratio range above 2.5. As utilization is raised in the electrode high fuel efficiency could be obtained by the shift of steam reforming reaction equilibrium as expected from the result of the calculations.