A heat treatment apparatus comprising: a cooling chamber; and a coolant passage defining body provided in the cooling chamber to define a coolant passage to supply a predetermined coolant to a workpiece passing through a conveyance path along a predetermined conveyance direction, wherein the coolant passage extends along a crossing direction which includes an up-down direction of the heat treatment apparatus, and the coolant passage is configured so that a cooling liquid as the coolant flows upward from a lower side in the coolant passage, wherein the coolant passage defining body includes a plurality of coolant passage defining members (2, 49,50), and wherein the plurality of coolant passage defining members (49,50) are configured to define the coolant passage in a state of housing the workpiece by being displaced to approach each other along a predetermined crossing direction crossing the conveyance direction, and configured to allow the workpiece to be let into and out of the coolant passage along the conveyance direction by being displaced to separate from each other along the crossing direction, wherein the crossing direction and the conveyance direction are disposed orthogonally to each other; wherein the coolant passage defining body includes, an upper member and a lower member as the plurality of coolant passage defining members, an introduction pipe, and a conveyance tray, the conveyance tray being disposed between the lower member and the upper member as the plurality of coolant passage defining members to convey the workpiece and to define a portion of the coolant passage, the conveyance tray cooperating with the lower member and the upper member, which is configured to define the coolant passage, and wherein the heat treatment apparatus further comprises a conveyance mechanism configured to convey the conveyance tray along the conveyance direction, and a vertical displacement mechanism configured to displace the upper member in the crossing direction relative to the lower member; wherein the vertical displacement mechanism is configured to displace the upper member to the lower member side such that the conveyance tray is sandwiched between the lower member and the upper member in the crossing direction. a cooling chamber; and a coolant passage defining body provided in the cooling chamber to define a coolant passage to supply a predetermined coolant to a workpiece passing through a conveyance path along a predetermined conveyance direction, wherein the coolant passage extends along a crossing direction which includes an up-down direction of the heat treatment apparatus, and the coolant passage is configured so that a cooling liquid as the coolant flows upward from a lower side in the coolant passage, wherein the coolant passage defining body includes a plurality of coolant passage defining members (2, 49,50), and wherein the plurality of coolant passage defining members (49,50) are configured to define the coolant passage in a state of housing the workpiece by being displaced to approach each other along a predetermined crossing direction crossing the conveyance direction, and configured to allow the workpiece to be let into and out of the coolant passage along the conveyance direction by being displaced to separate from each other along the crossing direction, wherein the crossing direction and the conveyance direction are disposed orthogonally to each other; wherein the coolant passage defining body includes, an upper member and a lower member as the plurality of coolant passage defining members, an introduction pipe, and a conveyance tray, the conveyance tray being disposed between the lower member and the upper member as the plurality of coolant passage defining members to convey the workpiece and to define a portion of the coolant passage, the conveyance tray cooperating with the lower member and the upper member, which is configured to define the coolant passage, and wherein the heat treatment apparatus further comprises a conveyance mechanism configured to convey the conveyance tray along the conveyance direction, and a vertical displacement mechanism configured to displace the upper member in the crossing direction relative to the lower member; wherein the vertical displacement mechanism is configured to displace the upper member to the lower member side such that the conveyance tray is sandwiched between the lower member and the upper member in the crossing direction.