To improve the heat-exchanging coefficient of a heat exchanger.

A heat exchanger has a metal case 1, a cooling pipe 3 being arranged in the metal case 1, a first separating outer shell 5 for closing one end of the cooling pipe 3, a second separating outer shell 6 for closing the other end of the cooling pipe 3, a heat generation body 2 being arranged in the cooling pipe 3, a painted layer 31 for absorbing infrared rays or far infrared rays being applied onto the inner wall of the cooling pipe 3, and a plurality of heat separators 4 being provided between the cooling pipe 3 and the heat generation body 2. An inlet 11 of water and an outlet 12 of water are formed in the metal case 1. The inside of the cooling pipe 3 is in vacuum. The painted layer 31 for absorbing infrared rays or far infrared rays absorbs the infrared rays or the far infrared rays and operates in vacuum.