To effectively anneal a cast iron pipe regardless of the difference in the cooling speed of the cast iron pipe by the bores, the kind of pipe, or sections of the pipe.

A heat treatment device for heat-treating a cast iron pipe 14 has a first furnace 15 which is used for the cementite decomposition of the cast iron pipe 14, a second furnace 16 which is used for the pearlite decomposition of the cast iron pipe 14, and a heat releasing zone 17 which is provided between these first furnace 15 and second furnace 16, along the carriage path 12 of the cast iron pipe 14. The second furnace 16 is constituted so that it can adjust the length of the heat releasing zone 17 by being able to come close to or go away from the first furnace 15 along a carriage path 12.