To understand the flow behavior of a melted polymer during injection molding with ultrasonic vibration, a new visualization device was developed and a method was proposed to analyze the filling velocity field during the injection molding process. Using the device and method, both the influence of ultrasonic vibration on the velocity of the filling front and the velocity distribution of a polypropylene melt within a cavity are discussed. The results showed that the filling front velocity of the melt was improved by ultrasonic vibration (the filling front velocity was improved by up to 27% when 200 W of ultrasonic power was applied). In addition, it was shown that the velocity gradient of the melt within the cavity decreased, and that the velocity distribution became more uniform during ultrasonic vibration.