An exhaust gas purification device is capable of diffusing and introducing a reducing agent into a NOx catalyst arranged in a downstream side exhaust gas passage. A housing of the device includes a main body in which an upstream side exhaust gas passage and the downstream side exhaust gas passage are housed, and a cap attached to a first surface of the main body 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. At least one of the first surface of the main body and a second surface in the cap which faces the first surface includes a guide portion, for guiding an exhaust gas flow, and a reducing agent diffusion portion, for facilitating mixing of the reducing agent with the exhaust gas.