Abstract: A spread spectrum modulation unit (12) performs spread spectrum clocking processing for a basic clock signal (BC) synchronized with the carrier frequency or its harmonic frequency of image information leaked from an unwanted electromagnetic wave. A modulation pattern generation unit (13) generates and outputs, as a modulation pattern signal (MP), a PN code having sign bit data synchronized with each pulse of the obtained spread spectrum clock signal SC. In addition, the modulation pattern generation unit (13) resets the repetition period of the PN code based on a horizontal sync signal (H). A modulated clock generation unit (14) modulates the spread spectrum clock signal (SC) in accordance with the modulation pattern signal (MP). The obtained modulated clock signal (MC) is amplified, generating a leakage prevention signal (JC). A leakage prevention signal containing a sideband component of a satisfactory level can be generated, obtaining a useful leakage prevention effect.