Thermal diffusion device

The utility model relates to a thermal diffusion device. A vapor chamber (1) is provided with: a frame (10) having a first inner wall surface (11a) and a second inner wall surface (12a) facing each other in the thickness direction; a working medium (20) sealed in the internal space of the housing (10); and a core body (30) disposed in the internal space of the frame body (10), the core body (30) having a portion in contact with a first inner wall surface (11a) and a second inner wall surface (12a) of the frame body (10) in a direction perpendicular to the thickness direction, and a steam flow path (50) formed in the internal space of the frame body (10). The planar shape of the housing (10) as viewed from the thickness direction has a first side (61) and a second side (62) that form an interior angle of 180 DEG or more, and the interior space of the housing (10) is further provided with a capillary structure (70) having a first portion (71) that is close to the first side (61) or the second side (62) and a se