PURPOSE: To carry out the shortening of a catalyst activating time, by saving of a catalyst activating gas and partial load operation, by diving a stock gas distribution chamber in a multi-tube type reactor into plural chambers to enable the partial activation of a catalyst.

CONSTITUTION: A stock gas introduced from a stock gas inlet 1b enters an upper gas distribution chamber 5b and, after reacted in a reaction tube 9b, the product gas is exhausted to the outside from a product gas outlet 2a through a lower gas distribution chamber 6a. At the same time, the product gas is introduced from a heating medium inlet 10 in order to receive or release the heat to be generated or absorbed in the reaction tubes 9a, 9b and taken out to the outside from a heating medium outlet 11 through a space formed by a reactor outer frame 7, a reactor mounting bed 8 and the reaction tubes 9a, 9b.