Abstract: A loose material catalyzer disposed in the exhaust pipe of an internal combustion engine for purifying the exhaust gases thereof. The catalyzer includes a housing having a cylindrical wall. A first perforated plate is disposed in the housing essentially perpendicular to the cylindrical wall thereof, and receives exhaust gases for purification. A second perforated plate is disposed in the housing essentially perpendicular to the cylindrical wall thereof and allows purified exhaust gases to proceed downstream. A catalyst bed is disposed in the housing and is bounded by the cylindrical wall thereof and the two perforated plates. The catalyst bed is provided with granular catalyst material that tends to disappear during use, whereby the first, upstream perforated plate is axially movably disposed in the housing to compensate for the disappearance of the catalyst material.