A thermoelectric conversion unit (2, 2D) comprising: a first flow path; a second flow path: a third flow path arranged between the first flow path and the second flow path; a first device arranged between the first flow path and the third flow path, the first device converting thermal energy given by a difference between temperature of a first fluid flowing through the first flow path and temperature of a third fluid flowing through the third flow path to electrical energy; a second device arranged between the second flow path and the third flow path, the second device converting thermal energy given by a difference between temperature of the second fluid flowing through the second flow path and the temperature of the third fluid to electrical energy; a first fin (40b, 40Cb) including a first bottom surface and a projecting portion, the first fin (40b, 40Cb) being arranged in the third flow path; and a second fin, including a second bottom surface and a recess (48, 48C) fitted to the projecting portion (44b, 44Ca), the second fin being arranged in the third flow path, wherein the first bottom surface is fixed to the first device directly or through a first member, the second bottom surface is fixed to the second device directly or through a second member, a distance between the first fin (40b, 40Cb) and the first device is shorter than a distance between the first fin (40b, 40Cb) and the second device, a distance between the second fin and the second device is shorter than a distance between the second fin and the first device, and a distance between the second bottom portion and the projecting portion (44b, 44Ca) is longer than a distance between the second bottom portion and the recessed portion (48, 48C). a first flow path; a second flow path: a third flow path arranged between the first flow path and the second flow path; a first device arranged between the first flow path and the third flow path, the first device converting thermal energy given by a difference between temperature of a first fluid flowing through the first flow path and temperature of a third fluid flowing through the third flow path to electrical energy; a second device arranged between the second flow path and the third flow path, the second device converting thermal energy given by a difference between temperature of the second fluid flowing through the second flow path and the temperature of the third fluid to electrical energy; a first fin (40b, 40Cb) including a first bottom surface and a projecting portion, the first fin (40b, 40Cb) being arranged in the third flow path; and a second fin, including a second bottom surface and a recess (48, 48C) fitted to the projecting portion (44b, 44Ca), the second fin being arranged in the third flow path, wherein the first bottom surface is fixed to the first device directly or through a first member, the second bottom surface is fixed to the second device directly or through a second member, a distance between the first fin (40b, 40Cb) and the first device is shorter than a distance between the first fin (40b, 40Cb) and the second device, a distance between the second fin and the second device is shorter than a distance between the second fin and the first device, and a distance between the second bottom portion and the projecting portion (44b, 44Ca) is longer than a distance between the second bottom portion and the recessed portion (48, 48C). a third flow path arranged between the first flow path and the second flow path; a first device arranged between the first flow path and the third flow path, the first device converting thermal energy given by a difference between temperature of a first fluid flowing through the first flow path and temperature of a third fluid flowing through the third flow path to electrical energy; a second device arranged between the second flow path and the third flow path, the second device converting thermal energy given by a difference between temperature of the second fluid flowing through the second flow path and the temperature of the third fluid to electrical energy; a first fin (40b, 40Cb) including a first bottom surface and a projecting portion, the first fin (40b, 40Cb) being arranged in the third flow path; and a second fin, including a second bottom surface and a recess (48, 48C) fitted to the projecting portion (44b, 44Ca), the second fin being arranged in the third flow path, wherein the first bottom surface is fixed to the first device directly or through a first member, the second bottom surface is fixed to the second device directly or through a second member, a distance between the first fin (40b, 40Cb) and the first device is shorter than a distance between the first fin (40b, 40Cb) and the second device, a distance between the second fin and the second device is shorter than a distance between the second fin and the first device, and a distance between the second bottom portion and the projecting portion (44b, 44Ca) is longer than a distance between the second bottom portion and the recessed portion (48, 48C).