PURPOSE: To form a hydrophilic coating film of an evaporator having a relatively high initial hydrophilicity and with the hydrophilicity hardly deteriorated with the lapse of time.

CONSTITUTION: An evaporator having a chromated substrate coating film is dipped in a soln. in which 0.3-2.0wt.% of a surfactant is added to 100wt.% of silica sol, and the evaporator is heated in an atmosphere kept at ≤100°C and dried. Otherwise, an evaporator having a chromated substrate coating film is dipped in a soln. in which 0.3-2.0wt.% of a surfactant and 0.4-0.8wt.% of 2-(4-thiazolyl)-benzimidazole are added to 100wt.% of silica gel, and the evaporator is heated in an atmopsphere kept at ≤100°C and dried.