To provide a catalyst for treatment of exhaust gas, provided with catalyst units with structure which surely prevents gas leakage without fixing catalyst unit frames to a catalyst block frame, and allows easy drawing-out or insertion of the catalyst units after assembling of the catalyst blocks without damaging catalyst elements.

The catalyst for treatment of exhaust gas comprises the catalyst blocks, formed by mounting a plurality of catalyst units formed by packing catalyst elements in catalyst unit frames 30, to a catalyst block frame, are loaded in a catalytic reactor through which the exhaust gas is circulated, and purifies harmful gas, wherein the catalyst unit frame 30 is a rectangular parallelepiped frame including catalyst unit side plates 32 restricting the exhaust gas flow direction, and is provided with gas leak preventive plates 34, 36 outwardly protruded from two adjacent sides at the upper end of an opening of the catalyst unit side plates 32.