A video display monitor, such as a plasma monitor, which uses a subfield method which overlaps weighted multiple binary video images in a time base for display. The stable driving of a plasma display panel may be assured and display in the 256 grey-level may be maintained although vertical synchronizing frequency of the input video signal changes. A vertical synchronizing measurement unit measures the vertical synchronizing frequency of the video signal, and a subfield number adjustment unit adjusts the number of subfields in accordance with a measured vertical synchronizing frequency. The stable driving of the plasma display panel and display in grey levels may be assured by selecting a ROM table with an output bit width equivalent to the number of subfields to be output from multiple ROM tables used for converting the number of bits in the input signal.