In situ mass spectroscopy with a capillary probe was conducted for the anode gas layer of a semi-passive direct methanol fuel cell (DMFC) employing a porous carbon plate (PCP) in order to evaluate the gas composition in contact with the anode. Different types of PCPs were used for the DMFC, and then the relationship between the gas composition in the gas layer and the current density was investigated. The profiles of the CO 2 gas pressure, methanol and water vapor pressures were discussed on the basis of the current density and the resistance for the methanol and CO 2 transport through the PCP. The current density linearly and identically increased with the increase in the partial pressure of methanol, P C H 3 OH , in the gas layer up to 7.5 kPa irrespective of the type of the PCP suggesting that the current was a function of P C H 3 OH and it was rate limited by the methanol transport to the anode. The calculated liquid methanol concentration equivalent to the measured gas mixture in the gas layer was about 5–7 M in the optimum conditions. This confirmed that the actual methanol activity on the anode of the DMFC with the PCP was controlled by the PCP and was similar to that of the usual liquid feed DMFC even when a very high concentration of methanol was in the reservoir.