Video transcoding technology is useful for seamless video service to guarantee quality of service (QoS). But video transcoding would distort the watermark signal embedded in the video content. In this paper, we propose a watermarking system that guarantees detecting watermark signal even after video transcoding. We analyzed the effect of video transcoding for watermark signal. Based on the analysis, metadata based approach is proposed to keep watermark in the video transcoding under QoS guaranteeing service environment. We performed experiments with widely used watermark algorithms to verify that the proposed watermarking system is useful.