To obtain a combustible gas which maintains calorific value inherent in a thermally cracked gas while conducted thermal decomposition normally as before.

This furnace has following mechanisms and steps: An organic waste A is fed into a furnace through a feeder 11 and moves in the direction of an outlet by heating with a cylindrical rotary furnace 1 heated from outside indirectly. In this step, the organic waste is thermally decomposed and the obtained thermally cracked gas is purged as a combustible gas B and the remained ungasified part is purged as a pyrolysis residue C from the outlet side. Steam or a combustible gas is led into a thermal decomposition zone of the cylindrical rotary furnace 1 to keep and control inner pressure positive and not to be invaded with outside air.