A joining structure comprising: a ceramic substrate; a metal member (1,7) made of copper, being a metal layer formed directly on the ceramic substrate or a metal layer formed directly on a metal thin film, the metal thin film being formed directly on the ceramic substrate; a metal porous body (2, 6) made of silver formed directly on the metal member (1, 7) and a semiconductor chip disposed on the metal porous body (2, 6), whereby the semiconductor chip is joined to the metal member (1, 7) via the metal porous body (2, 6), wherein the metal member (1, 7) consists of an inner layer and an outer layer directly on the inner layer, the first outer layer including at least one of main surfaces of the metal member (1,7) and being formed on the metal porous body side of the metal member (1,7), the inner layer being formed at a position farther from the metal porous body (2, 6) than the outer layer in a thickness direction and the metal porous body (2, 6) being joined directly to the first outer layer, characterized in that the outer layer has an average crystal grain size (ds) that is smaller than an average crystal grain size (di) of the inner layer, and the metal porous body (2, 6) has an average crystal grain size (dp) that is smaller than or equal to the average crystal grain size (ds) of the outer layer, said average crystal grain sizes being obtained in following manner: for obtaining the average crystal grain size (dp) of the metal porous body (2, 6), in an image, a straight line (a) is drawn in the middle of the thickness direction of the metal porous body (2, 6), and an average value of (length+ width)/2 for crystals existing on the straight line (a) is calculated; for obtaining the average crystal grain size (ds) of the outer layer, in an image, at a position at a distance of 1μm in the thickness direction from an interface between the metal porous body (2, 6) and the metal member (1, 7) to the metal member (1, 7) side, a line (c) is drawn along the interface, and an average value of (length+ breadth)/2 for crystals existing on the line (c) is calculated; and for obtaining the average crystal grain size (di) of the inner layer, in an image, a straight line (d) is drawn in the middle of the thickness direction of the metal member (1, 7), and an average value of (length+ breadth)/2 for crystals existing on the straight line (d) is calculated. a metal porous body (2, 6) made of silver formed directly on the metal member (1, 7) and a semiconductor chip disposed on the metal porous body (2, 6), whereby the semiconductor chip is joined to the metal member (1, 7) via the metal porous body (2, 6), wherein the metal member (1, 7) consists of an inner layer and an outer layer directly on the inner layer, the first outer layer including at least one of main surfaces of the metal member (1,7) and being formed on the metal porous body side of the metal member (1,7), the inner layer being formed at a position farther from the metal porous body (2, 6) than the outer layer in a thickness direction and the metal porous body (2, 6) being joined directly to the first outer layer, characterized in that the outer layer has an average crystal grain size (ds) that is smaller than an average crystal grain size (di) of the inner layer, and the metal porous body (2, 6) has an average crystal grain size (dp) that is smaller than or equal to the average crystal grain size (ds) of the outer layer, said average crystal grain sizes being obtained in following manner: for obtaining the average crystal grain size (dp) of the metal porous body (2, 6), in an image, a straight line (a) is drawn in the middle of the thickness direction of the metal porous body (2, 6), and an average value of (length+ width)/2 for crystals existing on the straight line (a) is calculated; for obtaining the average crystal grain size (ds) of the outer layer, in an image, at a position at a distance of 1μm in the thickness direction from an interface between the metal porous body (2, 6) and the metal member (1, 7) to the metal member (1, 7) side, a line (c) is drawn along the interface, and an average value of (length+ breadth)/2 for crystals existing on the line (c) is calculated; and for obtaining the average crystal grain size (di) of the inner layer, in an image, a straight line (d) is drawn in the middle of the thickness direction of the metal member (1, 7), and an average value of (length+ breadth)/2 for crystals existing on the straight line (d) is calculated.