The invention generally relates to a flexible heal exchanger comprising a pair of flexible multilayer thermoplastic polyimide films each of which comprises an aromatic polyimide substrate film showing no glass transition temperature or a glass transition temperature of 340.degree. C. or higher and a thermoplastic aromatic polyimide surface film showing a glass transition temperature in the range of 190 to 300.degree. C. in which the surface films of the flexible multilayer thermoplastic polyimide films face each other and are in part fused together, whereby producing between the flexible multilayer thermoplastic polyimide films a conduit pattern through which a fluid passes. The flexible multilayer thermoplastic polyimide films preferably have a thickness in the range of 10 to 125 .mu.m and comprise a linear expansion coefficient of MD, a linear expansion coefficient of TD and an average of linear expansion coefficients of MD and TD, in the range of 10.times.10.sup.-6 to 35.times.10.sup.-6 cm/cm/.degree. C. at 50-200.degree. C.