An evaporator includes a container, a wick provided in contact with an inner peripheral surface of said container and formed such that (1) if the number of pores per unit volume is fixed, the diameter of pores is varied, or (2) if the diameters of pores are formed substantially uniformly, the number of pores is varied, a sump having said wick as its inner wall surface and connected to a liquid pipe for supplying a liquid-phase working fluid, and a vapor channel formed in a contact surface of said container with respect to said wick so as to guide a gas-phase working fluid into a vapor pipe connected to an end portion of said container.