The flow visualization inside the oil film was tried using the method of photochromism. One of the photochromic dyes is dissolved in the PAO and used as a test solution for flow visualization and velocity measurement. The light source used is a 3rd harmonic of Nd-YAG laser. This ultra-violet light is shaped to a thin light sheet and irradiated to the oil film of test solution. The test oil of irradiated position is marked by a blue color and the movement of the mark was photographed by a high speed color video camera. The flow velocity was quantified from the obtained images and confirmed the validity of this technique by comparing to the velocity analyzed from the flow rate measurement. The usefulness for flow visualization was confirmed by demonstrating this technique to 3 different flow fields.