A heat-insulated pipe assembly (1) includes a pipe (2) allowing a fluid to flow therethrough, and a heat insulating member (3) covers at least a part of the pipe (2). The pipe (2) is used in an exhaust gas purification system using an SCR catalyst. The heat insulating member (3) has a closed-cell structure and suppresses an increase in a temperature of the fluid due to heat exchange with air. In the exhaust gas purification system using the SCR catalyst, the interior of the pipe (2) is thermally insulated from the exterior by the heat insulating member (3). Thereby an increase in the temperature of the fluid flowing through the pipe (2) due to heat exchange with air is suppressed.