The objective is to provide an internal-combustion-engine controller that can diagnose a smolder state in a subsidiary combustion chamber of a subsidiary-chamber-type internal combustion engine at low cost and in real time and that can perform control so as to securely produce a spark discharge. The controller controls an internal combustion engine in which a fuel-air mixture in the main combustion chamber is ignited with combustion gas to be injected through an orifice provided in a subsidiary combustion chamber; the controller includes an ignition plug and a detection probe arranged in the subsidiary combustion chamber, an ignition coil, a fuel injector, a smolder detector, and a smolder diagnosis device, and controls at least one of the ignition coil and the fuel injector in accordance with a diagnostic result in the smolder diagnosis device.