A filter element comprises an oil-impregnated filtering member, and a transmission
preventing layer formed on a downstream side of the filtering member, for restricting
transmission of at least the carbon dusts from the oil-impregnated filtering member.
The filtering member may be formed like a ridge-like-folded structure. The transmission
preventing layer may be formed of a coating layer having a resistance against a
viscous oil that is impregnated in the filtering member. This coating layer may
be formed uniformly as a whole or the thickness or the content of the coating layer
may be increased partially. A ventilation shutting-off member (filling layer) may
be formed as the transmission preventing layer in the root portion areas of the
ridge portions. The transmission preventing layer may comprise a resistant component
that is insoluble in the oil that is impregnated in the filtering member.
