A delta wing sets up self-excited rolling oscillation known as "wing rock" when it is placed in flow at high angles of attack. In this study, wind-tunnel tests of a slender-delta wing with 80-deg sweep angle at subsonic speeds were conducted. The phase-lock technique with a fast-responding Pressure-Sensitive Paint (PSP) has been employed to visualize unsteady pressure distribution on the oscillating model during the entire wing-rock cycle. Analysis of the PSP results together with roll angle measurement and high-speed Schlieren images has revealed that the lag in the leading-edge vortex displacement and the delay of vortex breakdown play an important role in rocking motion of the delta wing. In the present experiment, PSP has proved to be a powerful tool to study the interaction of vortices with a model in rocking motion. This technique will be used to analyze the driving mechanism of the wing rock and its controlin details.