To effectively determining whether the driver is in the specific state according to the use of the driving assistance device, provided is a driver monitoring device which is applied to a vehicle having a driving assistance device configured to execute a predetermined driving assistance control for assisting the driving of a driver. The driver monitoring device comprising a driver state acquisition unit configured to acquire a state of the driver; and a driver state determination unit configured to determine that the driver is in a specific state when the state of the driver acquired by the driver state acquisition unit satisfies a predetermined condition. The driver state determination unit is configured to relaxes the predetermined condition when the driving assistance device is executing the driving assistance control, as compared with a case where the driving assistance device is not executing the driving assistance control.