The automatic transmission fluid filter includes a plastic upper case having an outlet port, a plastic lower case having an inlet port , and a filter element including a filter medium and a plastic holder frame holding the periphery of the filter medium and held between the upper case and lower case, at least one of the upper case and lower case including a pair of joint portions respectively joined to opposite walls a of the holder frame, and a connecting wall connecting the pair of joint portions and having a circular arc longitudinal cross section protruding outward of one case.