PURPOSE: To prevent the early stage complete clogging of a filter flow passage, to improve the filtering efficiency and to maintain the filtering function over a long period of time by simple constitution in a filter device for mechanically separating sludge factors or the like from an original fluid that is caused to flow by pressure difference.

CONSTITUTION: This filter has a casing A having an inflow port 4 and an outflow port 5, an introducing cylinder B which is connected to the inflow port 4 at the lower end part thereof and extends upward inside the casing and has an expanded part 8 in the form of a horn at the upper end part thereof, a diffusion separating body C which is installed inside the casing above the introducing cylinder and has a filter surface in the form of an inverted cone inserted leaving a gap between it and the expanded part 8, a filter medium D fitted above the diffusion separating body at need, and a sedimentation part 10 formed of the peripheral wall and bottom part of the casing and the introducing cylinder and communicating with the gap.