Thermal diffusion device

The heat diffusion device comprises a frame body (10) which is provided with an internal space (13) and is provided with a first inner wall surface (11a) and a second inner wall surface (12a) which are oppositely arranged along a thickness direction (Z); a working medium (20) sealed in the internal space (13); a first core (35a) and a second core (35b) disposed in the internal space (13); and air-impermeable partition walls (14a, 14b) disposed in the internal space (13) and having a first end and a second end, the first core (35a) and the second core (35b) each having a portion in contact with a first inner wall surface (11a) and a second inner wall surface (12a) of the housing (10) in a direction perpendicular to the thickness direction, and a vapor flow path (50) formed in the internal space (13) of the housing (10). The second end portion of the partition wall is connected to the inner surface of the housing (10), and the partition wall is sandwiched between the first core and the second core when the inne