A refrigerant heat exchanger, comprising: a hollow container having a cylindrical shape; a plate stack disposed on an inner lower side of the hollow container, including plates each having a front side and a back side with a plurality of concavo-convex portions formed thereon which are stacked to form a first heat exchange flow passage through which a first refrigerant flows and a second heat exchange flow passage through which a second refrigerant flows; a supply pipe disposed in an interior space of the hollow container above the plate stack and configured to supply the first refrigerant to the plate stack; and a discharge pipe configured to exchange heat between the first refrigerant supplied from the supply pipe and the second refrigerant flowing through the plate stack and to discharge the first refrigerant, wherein a lower side of the plates of the plate stack has a semi-circular shape along and adjacent to an inner wall surface of the hollow container, wherein an upper side of the plates has a flattened shape having a greater curvature radius than a curvature radius of the semi-circular shape, wherein a second introduction hole which extends in a plate-stacking direction and into which the second refrigerant is introduced is disposed in an upper portion of the plate stack, and a second lead-out hole which extends in the plate-stacking direction and from which the second refrigerant is led out is disposed in a lower portion of the plate stack, wherein the second heat exchange flow passage is formed so as to extend and bend toward a side portion of the plate downward from the second introduction hole and to extend toward the second lead-out hole downward, in a view in the plate-stacking direction, and wherein the first heat exchange flow passage is formed so as to extend toward an end portion, with respect to a width direction, of the plate upward from the second lead-out hole, in the view in the plate-stacking direction. a hollow container having a cylindrical shape; a plate stack disposed on an inner lower side of the hollow container, including plates each having a front side and a back side with a plurality of concavo-convex portions formed thereon which are stacked to form a first heat exchange flow passage through which a first refrigerant flows and a second heat exchange flow passage through which a second refrigerant flows; a supply pipe disposed in an interior space of the hollow container above the plate stack and configured to supply the first refrigerant to the plate stack; and a discharge pipe configured to exchange heat between the first refrigerant supplied from the supply pipe and the second refrigerant flowing through the plate stack and to discharge the first refrigerant, wherein a lower side of the plates of the plate stack has a semi-circular shape along and adjacent to an inner wall surface of the hollow container, wherein an upper side of the plates has a flattened shape having a greater curvature radius than a curvature radius of the semi-circular shape, wherein a second introduction hole which extends in a plate-stacking direction and into which the second refrigerant is introduced is disposed in an upper portion of the plate stack, and a second lead-out hole which extends in the plate-stacking direction and from which the second refrigerant is led out is disposed in a lower portion of the plate stack, wherein the second heat exchange flow passage is formed so as to extend and bend toward a side portion of the plate downward from the second introduction hole and to extend toward the second lead-out hole downward, in a view in the plate-stacking direction, and wherein the first heat exchange flow passage is formed so as to extend toward an end portion, with respect to a width direction, of the plate upward from the second lead-out hole, in the view in the plate-stacking direction.