An exhaust gas purification device comprising: an exhaust gas passage having a return portion; a reducing agent supply portion which is arranged in the return portion or an exhaust gas passage on an upstream side of the return portion and which supplies a reducing agent into the exhaust gas passage; and a reduction catalyst which is arranged in an exhaust gas passage on a downstream side of the return portion and which reduces and purifies nitrogen oxide using the reducing agent; a housing of the exhaust gas purification device in which the upstream side exhaust gas passage and the downstream side exhaust gas passage are housed; and a cap which is attached to a first surface that is provided with an outlet side opening of the upstream side exhaust gas passage and an inlet side opening of the downstream side exhaust gas passage, characterized in that the return portion is formed by a space surrounded by the first surface and the cap, the housing includes a main body having said first surface, at least one of the first surface of the main body and a second surface in the cap which faces the first surface is provided with a guide portion, for guiding an exhaust gas flow, which is arranged in a direction intersecting with the first surface and the second surface, a diffusion plate as a reducing agent diffusion portion, for facilitating mixing of the reducing agent with the exhaust gas, which is arranged in the middle of the flow of the exhaust gas guided by the guide portion, a first ring-shaped member having a plurality of holes serving as opening portions in the circumferential direction, a second ring-shaped member having an opening portion that opens widely at one section in the circumferential direction, first guide plates and second guide plates, wherein, the first ring-shaped member is provided in the outlet side opening of the upstream side exhaust gas passage, the second ring-shaped member is provided in the inlet side opening of the downstream side exhaust gas passage, the first guide plates guide the exhaust gas to the diffusion plate, and the second guide plates guide the exhaust gas that has passed through the diffusion plate to the opening portion of the second ring-shaped member. an exhaust gas passage having a return portion; a reducing agent supply portion which is arranged in the return portion or an exhaust gas passage on an upstream side of the return portion and which supplies a reducing agent into the exhaust gas passage; and a reduction catalyst which is arranged in an exhaust gas passage on a downstream side of the return portion and which reduces and purifies nitrogen oxide using the reducing agent; a housing of the exhaust gas purification device in which the upstream side exhaust gas passage and the downstream side exhaust gas passage are housed; and a cap which is attached to a first surface that is provided with an outlet side opening of the upstream side exhaust gas passage and an inlet side opening of the downstream side exhaust gas passage, characterized in that the return portion is formed by a space surrounded by the first surface and the cap, the housing includes a main body having said first surface, at least one of the first surface of the main body and a second surface in the cap which faces the first surface is provided with a guide portion, for guiding an exhaust gas flow, which is arranged in a direction intersecting with the first surface and the second surface, a diffusion plate as a reducing agent diffusion portion, for facilitating mixing of the reducing agent with the exhaust gas, which is arranged in the middle of the flow of the exhaust gas guided by the guide portion, a first ring-shaped member having a plurality of holes serving as opening portions in the circumferential direction, a second ring-shaped member having an opening portion that opens widely at one section in the circumferential direction, first guide plates and second guide plates, wherein, the first ring-shaped member is provided in the outlet side opening of the upstream side exhaust gas passage, the second ring-shaped member is provided in the inlet side opening of the downstream side exhaust gas passage, the first guide plates guide the exhaust gas to the diffusion plate, and the second guide plates guide the exhaust gas that has passed through the diffusion plate to the opening portion of the second ring-shaped member.