An apparatus for purifying exhaust gas in combination with a vehicle body comprising a frame, comprising: a cylindrical first housing, which houses a diesel oxidation catalyst converter and a diesel particulate filter in this order in an exhaust gas flowing direction; and a cylindrical second housing, which houses a selective catalytic reduction converter for selectively reducing and purifying a nitrogen oxide in an exhaust gas by using ammonia, the first and the second housings being disposed on the outer side of the frame that extends in a longitudinal direction of a vehicle body, and an exhaust stream outlet port of the first housing and an exhaust stream inlet port of the second housing being communicatively connected through a communication pipe, wherein the first housing and the second housing are fixed to an outer surface of the frame through a bracket, characterized in that the first housing is configured to be dividable into two parts, which are a first casing that houses the diesel oxidation catalyst converter and a second casing that detachably houses the diesel particulate filter, and the first casing and the second casing are separably fastened through a plurality of first fastening members that are accessible from the outer side in the vehicle width direction, the exhaust stream inlet port of the first housing is formed by being bent inwards in the vehicle width direction and is detachably fastened, through a second fastening member accessible from the outer side in the vehicle width direction, to an exhaust pipe having a distal end thereof opened toward the outer side in the vehicle width direction, and wherein the first and the second housings are arranged in parallel such that an exhaust gas upstream end of each thereof is positioned on the outer side in a vehicle width direction. a cylindrical first housing, which houses a diesel oxidation catalyst converter and a diesel particulate filter in this order in an exhaust gas flowing direction; and a cylindrical second housing, which houses a selective catalytic reduction converter for selectively reducing and purifying a nitrogen oxide in an exhaust gas by using ammonia, the first and the second housings being disposed on the outer side of the frame that extends in a longitudinal direction of a vehicle body, and an exhaust stream outlet port of the first housing and an exhaust stream inlet port of the second housing being communicatively connected through a communication pipe, wherein the first housing and the second housing are fixed to an outer surface of the frame through a bracket, characterized in that the first housing is configured to be dividable into two parts, which are a first casing that houses the diesel oxidation catalyst converter and a second casing that detachably houses the diesel particulate filter, and the first casing and the second casing are separably fastened through a plurality of first fastening members that are accessible from the outer side in the vehicle width direction, the exhaust stream inlet port of the first housing is formed by being bent inwards in the vehicle width direction and is detachably fastened, through a second fastening member accessible from the outer side in the vehicle width direction, to an exhaust pipe having a distal end thereof opened toward the outer side in the vehicle width direction, and wherein the first and the second housings are arranged in parallel such that an exhaust gas upstream end of each thereof is positioned on the outer side in a vehicle width direction.