An On-Screen-Display (OSD) insert circuit includes an OSD signal generator for generating a switching signal and an analog additional image signal based on a first clock signal, an analog-to-digital converter for converting the generated additional image signal into a digital additional image signal based on a second clock signal as a sampling clock signal, a switching circuit for switching between a digital video signal and the digital additional image signal based on the switching signal, and a control-signal generator operable to generate the second clock signal. The digital video signal has a horizontal synchronizing signal. The control-signal generator is operable to reset the first clock signal with a signal having a predetermined phase difference including zero with reference to the horizontal synchronizing signal, and to generate the second clock signal so that a phase of the second clock signal is adjusted with respect to the horizontal synchronizing signal. This OSD insert circuit can insert the analog additional image signal into the digital video signal without converting the digital video signal into an analog signal.