The cavitation phenomenon in He II flow through a two-dimensional Venturi channel was investigated by visualization and pressure measurement. Liquid helium flow was made by shrinkage of bellows pump, and the cavitation was induced in the flow at the throat of the channel. The overview of cavitation can be observed through the optical windows of the cryostat, and was recorded by a high-speed video camera and a still camera. The pressure loss due to the flow through the channel was measured by a differential pressure transducer. By analyzing the pressure loss data, the static pressure at the throat of the channel could be estimated. Through this experiment, it was found that cavitation in He II flow has some unique characteristics different from ordinary fluids. Once cavitation is induced in an He II flow, it jumps suddenly to fully developed scale without the weak cavitation stage seen in other fluids.