A heat exchanger comprising: (a) a first member including a main body and a heat transfer accelerating portion formed on the main body as one unit, the main body having a lower surface and an upper surface configured to receive an object to be heat-exchanged; and (b) a second member having an upper surface as a reference surface and a lower surface including a recess indented in the downward direction from the reference surface and functioning as a flow channel, wherein the flow channel has a plurality of straight line portions (521a to 521d) parallel to each other and turn portions (522a to 522c)which fluidically communicates end portions of the straight line portions (521a to 521d), and wherein (c) the first member and the second member are joined with each other at the reference surface in a state where the heat transfer accelerating portion of the first member is inserted into the recess of the second member, wherein the recess is divided by wall portions (511, 512, 513) of the lower case to form said plurality ofstraight line portions (521 a to 521 d), characterized in that (d) the second member is provided on the lower surface with a projecting portion (5d, 5e, 5c, 5g, 5h) that projects in a downward direction from an end portion of the lower surface, wherein (e) the turn portions are (522a - 522c) formed in the projecting portion (5d, 5e, 5c, 5g, 5h), the straight line portions (521a - 521 d) having a bottom, the turn portion s (522a to 522c) having a bottom and fluidically communicating end portions of the adjacent straight line portions (521a - 521d) to turn a flow direction between the adjacent straight line portions (521a - 521d), and wherein (f) a depth (B, H) between the reference surface and the bottom of the turn portions (522a - 522c) is set to be larger than depths between the reference surface and the bottoms of the straight line portions (521a-521d), in an area of the turn portions (522a - 522c) fluidically extending from an outermost end portion of one of the adjacent straight line portions (521a-521d) to an outermost end portion of the other adjacent straight line portions (521 a-521 d) in such a way that cooling medium lows in the downward direction from the one of the adjacent straight line portions (521 a-52 I d), then in a horizontal direction perpendicular to the downward direction, and then in an upward direction to other of the adjacent straight line portions (521a-521d) in the turn portions (522a-522c) wherein the wall portion at the center of the heat exchanger is offset in a lateral direction of the lower case at its intermediate portion which is gradually slanted along the longitudinal direction so that cooling water can smoothly change its flow volumes and flow speed (a) a first member including a main body and a heat transfer accelerating portion formed on the main body as one unit, the main body having a lower surface and an upper surface configured to receive an object to be heat-exchanged; and (b) a second member having an upper surface as a reference surface and a lower surface including a recess indented in the downward direction from the reference surface and functioning as a flow channel, wherein the flow channel has a plurality of straight line portions (521a to 521d) parallel to each other and turn portions (522a to 522c)which fluidically communicates end portions of the straight line portions (521a to 521d), and wherein (c) the first member and the second member are joined with each other at the reference surface in a state where the heat transfer accelerating portion of the first member is inserted into the recess of the second member, wherein the recess is divided by wall portions (511, 512, 513) of the lower case to form said plurality ofstraight line portions (521 a to 521 d), (d) the second member is provided on the lower surface with a projecting portion (5d, 5e, 5c, 5g, 5h) that projects in a downward direction from an end portion of the lower surface, wherein (e) the turn portions are (522a - 522c) formed in the projecting portion (5d, 5e, 5c, 5g, 5h), the straight line portions (521a - 521 d) having a bottom, the turn portion s (522a to 522c) having a bottom and fluidically communicating end portions of the adjacent straight line portions (521a - 521d) to turn a flow direction between the adjacent straight line portions (521a - 521d), and wherein (f) a depth (B, H) between the reference surface and the bottom of the turn portions (522a - 522c) is set to be larger than depths between the reference surface and the bottoms of the straight line portions (521a-521d), in an area of the turn portions (522a - 522c) fluidically extending from an outermost end portion of one of the adjacent straight line portions (521a-521d) to an outermost end portion of the other adjacent straight line portions (521 a-521 d) in such a way that cooling medium lows in the downward direction from the one of the adjacent straight line portions (521 a-52 I d), then in a horizontal direction perpendicular to the downward direction, and then in an upward direction to other of the adjacent straight line portions (521a-521d) in the turn portions (522a-522c) wherein the wall portion at the center of the heat exchanger is offset in a lateral direction of the lower case at its intermediate portion which is gradually slanted along the longitudinal direction so that cooling water can smoothly change its flow volumes and flow speed