A filter device provided with a filter and a rotation seal, the filter device removing foreign matter contained in fluid flowing through the annular gap between a shaft and a housing, which rotate relative to each other, wherein the device is compact, has both pressure resistance and low friction torque, and facilitates the replacement of the filter. A filter device (100) is provided with an annular filter (110) which is provided to a shaft (10) and a rotation seal (120) which seals between the filter (110) and a housing (20). The rotation seal (120) is provided with an annular magnet (140) which is provided to the filter (110) and a flexible annular seal member (130) which is provided to the housing (20). The seal member (130) is provided upstream of the magnet (140) and has an annular facing portion (132) which axially faces the magnet (140), is impregnated with a magnetic fluid (150), and is in contact with the magnet (140).