To prevent a polyester film from being embrittled under the substutitute refrigerant/synthetic oil atmosphere in an electric motor for a refrigerant compressor by setting the dehumidifying speed to a specified value or less.

An electrical polyester film has dehumidifying rate of 98%/hr. or lower. A cavity contained structure taking incompatible thermoplastic resin as a core is formed on polyester inside the film, it is desirable that polyalkylene glycol or a copolymer/derivatives of it be not substantially contained in the film, and the permittivity of the film is preferably not more than 2.9. The mixing amount of the thermoplastic resin incompatible with cavity forming agent, namely, polyester in relation to polyester is different according to the target amount of cavities, however, the range of 3 to 20 wt.% in relation to the whole film is preferable, more preferably, the range of 5 to 18 wt.%. Moreover the moisture regain of the polyester film is preferably less than or equal to 100 ppm.