An on-screen display (OSD) signal generating circuit which provides an analog output is utilized for realizing on-screen display of a digital display device and providing stable operations and signals. An OSD signal processing device has an OSD signal generating circuit which outputs an analog OSD signal in the form of a current and a resistor which has its one end connected to the output terminal of the OSD signal generating circuit and its other end connected to ground to convert a current signal into a voltage signal. In an analog-to-digital converter the input terminal of which is connected to the output terminal of the OSD signal generating circuit, the sampling upper- and lower-limiting voltages of a signal applied to the input terminal are defined by a power source voltage and ground potential, respectively.