Passive production of synthesis gas using a porous material block containing liquid methanol has been investigated. A penetrating hole prepared in the block, around which the catalyst is supported, is heated using a wire coil heater that is set up so as to contact with the hole-surface. Evaporation, catalytic action, and liquid supply due to capillary suction are induced due to the heat transfer into the porous structure, resulting in the successive production of the decomposed gases. However, the capillary suction suppresses the extension of a dried region and the temperature increase in a catalyst-supported region to the reaction level. Theoretical analysis of the reacting gas flow in the catalyst-supported region suggests that the yield is significantly dependent on the catalyst temperature, which increases the reaction rate exponentially, and on the thickness of the catalyst-supported region. Separation of the catalyst-supported region from the liquid containing region by a small gap suppresses excessive evaporation and raises temperature over the catalyst-supported region, resulting in the significant increases in the yield and the response at smaller heating rates.