PURPOSE: To provide data multiplexing method and device capable of effectively attaining multiplex transmission without generating disturbance on a screen.

CONSTITUTION: A level detecting means 6 outputs a gate signal for distinguishing the flyback time of an input video signal from a video period and a level signal corresponding to a luminance signal amplitude in the video period of the input video signal. An amplitude/speed converting means 7 converts the amplitude and speed of input data based upon the gate signal and the level signal outputted from the means 6. A digital modulation means 4 executes the digital modulation of an orthogonal carrier orthogonally intersecting with the phase of a video carrier based upon the data modulated by the means 7. A VSB modulation means 2 executes the vestigial side-band modulation of the video carrier by the input video signal. A synthesizing means 5 synthesizes the video carrier modulated by the means 2 and the orthogonal carrier modulated by the means 4.