To provide a heat exchanger having high heat exchanging efficiency and small size for its heat exchanging quantity, a compact heat pump and a dehumidifier arranged compactly.

The heat exchanger comprises a first section 310 for supplying a first fluid A, a second section 320 for supplying a second fluid B, first fluid channels 251A, B, C for supplying a third fluid exchanging heat with the first fluid A flowing through the first section 310, and second fluid channels 252A, B, C for supplying a third fluid exchanging heat with the second fluid B flowing through the second section 320. The third fluid flows from the first fluid channels 251A, B, C through the second fluid channels 252A, B, C and evaporates, at a specified pressure, on the channel side heat transfer surface of the first fluid channels 251A, B, C and condensates, at a specified pressure, on the channel side heat transfer surface of the second fluid channels 252A, B, C. The third fluid condensed in the second fluid channels 252A, B, C can be supplied reversely between the first fluid channels 251A, B, C and the second fluid channels 252A, B, C.