The utility model provides a monitoring device and a vehicle-mounted electronic control device which guarantee higher failure protection performance when used in an action monitoring structure of a control device such as a microcomputer or the like by using charging and discharging of a capacitor. The charging circuit of the monitoring device charges a capacitor by providing first current via a first path, while the discharging circuit of the monitoring device correspondingly and periodically inputs a monitoring signal to the control device in order to discharge the capacitor. When charging voltage of the capacitor reaches a specified value, a signal output circuit outputs a signal indicating absence of the monitoring signal in order to constrain the charging circuit to charge the capacitor by providing second current via a second path under the condition that the first current of the charging circuit is unsupplied. The second current is larger than the first current while the second path is different from the first path. The vehicle-mounted electronic control device comprises the monitoring device, a microcomputer, and a capacitor.