The present invention is directed to a method for producing a fluid treatment device having a honeycomb member in a metallic cylindrical housing with a shock absorbent member wrapped around the honeycomb member. The method comprises the steps of (1) inserting the honeycomb member with the shock absorbent member wrapped around the honeycomb member, into the cylindrical housing, (2) forming a necking portion on at least one end portion of the cylindrical housing, with a body portion thereof being clamped, and (3) reducing a diameter of at least a part of the cylindrical housing with the shock absorbent member received therein, together with the shock absorbent member, to such an extent that a desired inner diameter of the part of the cylindrical housing is provided enough to cause the shock absorbent member to produce a desired holding pressure for holding the honeycomb member in the cylindrical housing.