An exhaust gas purification system for an internal combustion engine, comprising: a filter provided for an exhaust gas passage of the internal combustion engine and configured to collect particulate matter in an exhaust gas; PM detecting means provided on a downstream side of the filter in the exhaust gas passage, and configured to collect the particulate matter in the exhaust gas allowed to pass through the filter between electrodes by applying a voltage between the electrodes, and configured to output a signal corresponding to an amount of collected particulate matter; and filter malfunction judging means configured to judge a malfunction of the filter on the basis of the output of the PM detecting means at a predetermined time point during the collection of the particulate matter, characterized in that the filter malfunction judging means are further configured so that the voltage applied between the electrodes is changed during the collection of the particulate matter by the PM detecting means depending on at least one of a concentration of the particulate matter in the exhaust gas on the downstream side of the filter, a flow rate of the exhaust gas on the downstream side of the filter, and an exhaust gas temperature difference as a difference between a temperature of the exhaust gas on the downstream side of the filter and a temperature of the PM detecting means so that: the voltage applied between the electrodes is lowered if the concentration of the particulate matter in the exhaust gas on the downstream side of the filter is higher, as compared with if the concentration of the particulate matter is lower; the voltage applied between the electrodes is lowered if the flow rate of the exhaust gas on the downstream side of the filter is larger, as compared with if the flow rate of the exhaust gas is smaller; and/or the voltage applied between the electrodes is lowered if the exhaust gas temperature difference as the difference between the temperature of the exhaust gas on the downstream side of the filter and the temperature of the PM detecting means is larger, as compared with if the exhaust gas temperature difference is smaller. a filter provided for an exhaust gas passage of the internal combustion engine and configured to collect particulate matter in an exhaust gas; PM detecting means provided on a downstream side of the filter in the exhaust gas passage, and configured to collect the particulate matter in the exhaust gas allowed to pass through the filter between electrodes by applying a voltage between the electrodes, and configured to output a signal corresponding to an amount of collected particulate matter; and filter malfunction judging means configured to judge a malfunction of the filter on the basis of the output of the PM detecting means at a predetermined time point during the collection of the particulate matter, characterized in that the filter malfunction judging means are further configured so that the voltage applied between the electrodes is changed during the collection of the particulate matter by the PM detecting means depending on at least one of a concentration of the particulate matter in the exhaust gas on the downstream side of the filter, a flow rate of the exhaust gas on the downstream side of the filter, and an exhaust gas temperature difference as a difference between a temperature of the exhaust gas on the downstream side of the filter and a temperature of the PM detecting means so that: the voltage applied between the electrodes is lowered if the concentration of the particulate matter in the exhaust gas on the downstream side of the filter is higher, as compared with if the concentration of the particulate matter is lower; the voltage applied between the electrodes is lowered if the flow rate of the exhaust gas on the downstream side of the filter is larger, as compared with if the flow rate of the exhaust gas is smaller; and/or the voltage applied between the electrodes is lowered if the exhaust gas temperature difference as the difference between the temperature of the exhaust gas on the downstream side of the filter and the temperature of the PM detecting means is larger, as compared with if the exhaust gas temperature difference is smaller. the voltage applied between the electrodes is lowered if the concentration of the particulate matter in the exhaust gas on the downstream side of the filter is higher, as compared with if the concentration of the particulate matter is lower; the voltage applied between the electrodes is lowered if the flow rate of the exhaust gas on the downstream side of the filter is larger, as compared with if the flow rate of the exhaust gas is smaller; and/or the voltage applied between the electrodes is lowered if the exhaust gas temperature difference as the difference between the temperature of the exhaust gas on the downstream side of the filter and the temperature of the PM detecting means is larger, as compared with if the exhaust gas temperature difference is smaller.