To provide an oil filter of a transmission for surely capturing contamination according to the grain size without deteriorating efficiency of an oil pump while restraining a pressure loss in an oil flow.

This oil filter 70 has a grain size distribution gauge 64 for measuring the grain size distribution of an impurity included in oil flowing in an oil passage 50 and a plurality of filter parts 60a, 60b, 60c and 60d different in roughness of meshes, and has a filter 60 for filtering the impurity included in the oil flowing in the oil passage 50 by interposing one of the filter parts in the oil passage 50 and a control device 62 for selectively interposing one of the filter parts in the oil passage 50 according to the grain size distribution measured by the grain size distribution gauge 64.