To solve such a problem that a filter with a generally employed filter medium greatly depends on the performance of the filter medium, thus not being suitable for filtering high-viscosity oil, and has the limit of its use even when warmed up for filtration.

A filter for oil includes a coil part giving a modulation electromagnetic field to the oil, a modulation electromagnetic field generator giving an AC power source which generates the modulation electromagnetic field to the coil part, and a filter filtering the oil to which the modulation electromagnetic field is given.