A heat treatment vessel coupler for coupling together heat treatment vessels adjacent in a horizontal direction, of a plurality of rectangular box-shaped heat treatment vessels each loaded with a material to be treated, conveyed in a conveying direction on a plurality of rotationally driven conveyor rollers arranged in parallel inside a heat treatment furnace, the coupler comprises a plate body smaller than each of the heat treatment vessels, at least a pair of engagement protrusions each projecting from the plate body and engaging with a corresponding one of side walls of the heat treatment vessels, and a spacer protrusion projecting from the plate body between the at least a pair of engagement protrusions so as to be sandwiched between the heat treatment vessels with which the at least a pair of engagement protrusions engage respectively.