Distribution of a precoat agent is made uniform in a target gas directing to a filter. To this end, an intake opening of a filter device is formed as a slit-like or rectangular opening extending in a width direction. An accumulation recess is formed in an upper edge portion of the intake opening to have a profile that opens downward as viewed from a longitudinal direction of the intake opening. The accumulation recess is continuously formed in the longitudinal direction of the intake opening. A precoat agent nozzle is provided in a longitudinal predetermined position of the intake opening for ejecting the precoat agent and carrier gas toward an inner depth surface of the accumulation recess.