In a malfunction diagnosis device for an exhaust gas purification system including an exhaust gas purification catalyst disposed in the exhaust passage of an internal combustion engine, and a reductant supply device that supplies reductant to exhaust gas that flows in an upstream side of the exhaust gas purification catalyst, the degree of degradation of the exhaust gas purification catalyst is diagnosed by comparing the timing at which the bed temperature of the exhaust gas purification catalyst starts to rise due to the heat of moisture adsorption reaction, against a normal value.