PURPOSE: To suppress formation of carbon, sustain catalytic activity and efficiently and continuously produce mixed gas of H2-CO, by thermally decomposing methanol containing a specific amount of water added thereto in the presence of an Ni-K or Cu-Cr-Mn based catalyst.

CONSTITUTION: An Ni-K based catalyst activated by heating at about 350°C for about 30hr in a 100% H2 atmosphere or Cu-Cr-Mn based catalyst obtained by activating components of 19wt% CuO, 23wt% Cr2O3, 15wt% MnO2 and 25wt% Al2O3 in an atmosphere containing H2 in 10vol% based on N2 at about 460°C for about 16hr is filled in a catalyst cylinder 5. Methanol 1 containing 1W5vol% water and an inert gas 2, e.g. N2, are introduced into a blender 3 and blended. The resultant blend is then fed through a pipe 4 into the above- mentioned catalyst cylinder 5, thermally decomposed by heating at 200W600°C in a heater 6 to give a mixed gas consisting essentially of CO and H2, which is then fed into a heat-treating furnace 8.