A stick-slip experiment is performed to investigate generation mechanism for high-frequency ground motion at immediate proximity to a fault. A granite sample with a 40cm long pre-cut fault is compressed by a bi-axial loading apparatus to generate unstable slips on the fault. Ground motion at frequencies from 200kHz to 1MHz is measured with a wide-frequency band AE sensor. The observed records reveal some characteristics of high-frequency ground motion as follows: (1) The amplitude of ground motion strongly depends on the rupture velocity. (2) Near-fault strong-motion duration is much shorter than slip duration and well correlated with the local breakdown time. (3) Near-fault strong motion contains much higher frequencies than those expected from the breakdown zone size. These results indicate that near-fault high-frequency strong motion is generated by irregular rupture during the local breakdown process. The result (2) suggests that the breakdown zone size for an earthquake can be estimated from near-fault high-frequency strong motion records.