PURPOSE: To effectively utilize holding heat of an empty tray and to save energy of a tempering furnace by carrying the empty tray passed in a heating furnace together with the tray loading a work into the tempering furnace after by-passing a quenching vessel.

CONSTITUTION: The tray 14 loading the work W is passed through the heating furnace 10, and after heating the work W to the prescribed temp., the tray 14 is shifted an elevator basket 25 through an ejecting vestibule 19. In there, after executing the quenching treatment by dipping the work W into the quenching vessel 11, the above tray 14 is sent to the tempering furnace 12 through a first driving roller conveyor 30, to execute tempering treatment. In the heat treatment apparatus, the above tray 14 and empty tray 15 heated at random are detected with a photoelectric detector 26 in the heating furnace 10 and the empty tray 15 is by-passed to the quenching vessel 11 and pushed out on an elevator 32 from a selective conveyor 27. Successively, this empty tray 15 is carried into the above tempering furnace 12 with a second driving roller conveyor 33, and while carrying the empty tray with a driving roller conveyor 34 at lower step in the furnace, this holding heat is effectively utilized to the tempering heating.