A structure of a heat exchanger core without a header plate, in which: a pair of plates (5, 6) having a pair of side walls (1, 2) extended upward on both sides, respectively, to form a whole body into a groove shape, and bulging portions formed at edges on both open sides of a groove bottom on an outer side in a thickness direction orthogonally to the side walls (1, 2) are fitted with each other facing in a reverse direction, to thereby form a flat tube; and a casing is fitted onto an outer periphery of a stacked body configured by stacking a plurality of the flat tubes on each other at the bulging portions, wherein: an external frame portion whose inner periphery matches with an outer periphery on a bulging portion side of the stacked body, an inside flange portion formed at a peripheral edge of the external frame portion, and a packing holding portion extended upward in a side wall direction at an inner edge of the inside flange portion constitute a frame body; characterised in that the external frame portion of a pair of the frame bodies are fitted onto both ends on a bulging portion side of the stacked body to grasp outer peripheries at both end portions of the stacked body with the frame body, and the casing is fitted onto an outer periphery of the external frame portion of the frame body; and as a result, an annular groove into which a packing for seal can be inserted is formed between a packing grasping portion of the frame body and both end edges of the casing. a pair of plates (5, 6) having a pair of side walls (1, 2) extended upward on both sides, respectively, to form a whole body into a groove shape, and bulging portions formed at edges on both open sides of a groove bottom on an outer side in a thickness direction orthogonally to the side walls (1, 2) are fitted with each other facing in a reverse direction, to thereby form a flat tube; and a casing is fitted onto an outer periphery of a stacked body configured by stacking a plurality of the flat tubes on each other at the bulging portions, wherein: an external frame portion whose inner periphery matches with an outer periphery on a bulging portion side of the stacked body, an inside flange portion formed at a peripheral edge of the external frame portion, and a packing holding portion extended upward in a side wall direction at an inner edge of the inside flange portion constitute a frame body; characterised in that the external frame portion of a pair of the frame bodies are fitted onto both ends on a bulging portion side of the stacked body to grasp outer peripheries at both end portions of the stacked body with the frame body, and the casing is fitted onto an outer periphery of the external frame portion of the frame body; and as a result, an annular groove into which a packing for seal can be inserted is formed between a packing grasping portion of the frame body and both end edges of the casing. an external frame portion whose inner periphery matches with an outer periphery on a bulging portion side of the stacked body, an inside flange portion formed at a peripheral edge of the external frame portion, and a packing holding portion extended upward in a side wall direction at an inner edge of the inside flange portion constitute a frame body; characterised in that the external frame portion of a pair of the frame bodies are fitted onto both ends on a bulging portion side of the stacked body to grasp outer peripheries at both end portions of the stacked body with the frame body, and the casing is fitted onto an outer periphery of the external frame portion of the frame body; and as a result, an annular groove into which a packing for seal can be inserted is formed between a packing grasping portion of the frame body and both end edges of the casing.