A denitration reaction container (11) in which an evaporation chamber (23) that infuses urea water or ammonia water into exhaust gas, and a reaction chamber (25) that brings the exhaust gas into contact with catalytic units (24), are arranged integrally with a partition wall (21) therebetween. The partition wall (21) is provided with a thermal-expansion-absorbing structure, which is a dual-wall structure comprising a main partition plate (52) and an auxiliary partition plate (53) arranged with a fixed gap therebetween, and which absorbs differences in thermal expansion in the direction of the horizontal section of the reaction container (11), with axial direction slide mechanisms (55) being provided between the hull body (20B) and the main partition plate (52) and the auxiliary partition plate (53) to absorb differences in thermal expansion in the direction of the central axis (O); of the container. Furthermore, the main partition plate (52) is provided with a movable seal part having an elastic seal plate that makes sliding contact while moving with the displacement of the auxiliary plate (53) due to thermal expansion, thereby preventing blow-by of the exhaust gas with respect to the gap (54) between the main partition plate (52) and the auxiliary partition plate (53).