The mechanism of lateral contamination in developed turbulent wedge was examined experimentally. Turbulent wedge was generated by a cluster of cylinder roughness elements, and high-resolution hot-wire rakes and PIV were employed to capture and identify turbulent structures developing in the laminar-turbulent interface of the developed turbulent wedge. The results showed that oblique fin-like turbulent structures with oblique (swept-back) angle of about 75 \(^\circ \) developed in the region away from the wall. Occurrence of fin structures were rather periodic and their frequency gradually decreased downstream, suggesting that a certain frequency selection mechanism exists there. The fin structures caused development and breakdown of near-wall low-speed streaks in the ambient laminar region. Thus, we concluded that development of fin structures was responsible for lateral growth of developed turbulent wedge.