In a conventional geodetic VLBI technique, a number of frequency channels in an IF band signal are converted into video-frequency-band signals ranging 0 to several MHz. They are utilized for a band width synthesis to determine the observed delay with a high accuracy. An equipment that converts an IF signal into a video signals is called a video converter. An image rejection mixer is used in the video converter to obtain a upper side band and lower side band signals, independently. Video signals are lastly converted into digital signals by an input interface unit. The input interface has a suitable range of input signal level to work properly. Therefore the level of video signals should be adjusted to satisfy the input level condition of the input interface unit. To utilize this purpose, the video converter is equipped with an output level monitor. By using this function, it is possible to calibrate the correlation amplitude of radio source. In this papeer, we will describe a calibration method using the readout of video-converter-output level, and results actually applied for the survey of VLBI antenna blocking.