An exhaust gas purification device of an internal combustion engine, comprising: a filter that is provided in an exhaust passage of the internal combustion engine and configured to collect particulate matter in an exhaust gas; urea water spraying means that is provided in the exhaust passage downstream of the filter and configured to spray urea water into the exhaust gas; a selective reduction catalyst that is provided in the exhaust passage downstream of the urea water spraying means and configured to reduce and purify a nitrogen compound in the exhaust gas by using ammonia produced from the urea water; electrostatic capacity detecting means (17a, 17b) configured to detect an electrostatic capacity of the filter; filter temperature estimating means configured to estimate inside temperature of the filter based on the detected electrostatic capacity; and filter regenerating means configured to carry out forced regeneration by supplying fuel to the filter and raising temperature of the filter to or over burning temperature of the particulate matter, the filter regenerating means being configured to carry out the forced regeneration with an amount of fuel that is supplied to make the inside temperature equal to a predetermined target lower limit temperature, if the calculated inside temperature is equal to or higher than activation temperature of the selective reduction catalyst, the target lower limit temperature being temperature to start burning of the particulate matter, and the filter regenerating means being configured to carry out the forced regeneration with another amount of fuel that is supplied to make the inside temperature equal to a predetermined target upper limit temperature, if the calculated inside temperature is lower than the activation temperature of the selective reduction catalyst, the target upper limit temperature being temperature to avoid erosion of the filter. a filter that is provided in an exhaust passage of the internal combustion engine and configured to collect particulate matter in an exhaust gas; urea water spraying means that is provided in the exhaust passage downstream of the filter and configured to spray urea water into the exhaust gas; a selective reduction catalyst that is provided in the exhaust passage downstream of the urea water spraying means and configured to reduce and purify a nitrogen compound in the exhaust gas by using ammonia produced from the urea water; electrostatic capacity detecting means (17a, 17b) configured to detect an electrostatic capacity of the filter; filter temperature estimating means configured to estimate inside temperature of the filter based on the detected electrostatic capacity; and filter regenerating means configured to carry out forced regeneration by supplying fuel to the filter and raising temperature of the filter to or over burning temperature of the particulate matter, the filter regenerating means being configured to carry out the forced regeneration with an amount of fuel that is supplied to make the inside temperature equal to a predetermined target lower limit temperature, if the calculated inside temperature is equal to or higher than activation temperature of the selective reduction catalyst, the target lower limit temperature being temperature to start burning of the particulate matter, and the filter regenerating means being configured to carry out the forced regeneration with another amount of fuel that is supplied to make the inside temperature equal to a predetermined target upper limit temperature, if the calculated inside temperature is lower than the activation temperature of the selective reduction catalyst, the target upper limit temperature being temperature to avoid erosion of the filter.