According to the invention, a particulate matter sensor is installed in an exhaust passage of an internal-combustion engine. A control unit for this internal-combustion engine detects a particulate amount in an exhaust gas through the exhaust passage in response to an output from the particulate matter sensor. Further, the control unit for the internal-combustion engine forms a particulate layer on electrode surfaces of the particulate matter sensor by applying a particulate capturing voltage between the electrodes during a first period. Further, the control unit maintains the formed particulate layer during a second period. It is noted here that the phrase “maintain the formed particulate layer” includes the meanings “maintaining the formed particulate layer as it is” and “inhibiting control to remove the particulate layer”.