Longitudinal ultrasonic waves propagating at a natural crack were experimentally observed using the photoelasticity with sensitive tint technique. A natural crack in a glass plate was induced by thermal stress and crack opening displacement δ was evaluated as δ < 0.2μm by optical interferometry. Visualized images showed that the tensile phase of ultrasonic waves was reflected from the surface of crack, whereas, the compressional phase of ultrasonic waves was transmitted through the crack. These ultrasonic properties demonstrated the principle of higher harmonic wave generation at closed crack in nonlinear ultrasonics. In addition, visualized images of multiple ultrasonic waves passing through the crack were captured, and frequency analysis was performed using the luminance distribution. The results showed that in the region where the ultrasonic waves just passed the crack, one half component of the incident ultrasonic frequency appeared strongly.