This paper proposes a contention-free burst scheduling scheme in optically burst-switched WDM networks. We construct contention-free wavelength planes (lambda-planes) by assigning dedicated wavelengths to each ingress node. Bursts are transmitted to their egress nodes on lambda-planes, along routes forming a spanning tree. As a result, contention at intermediate core nodes is completely eliminated, and contention at ingress nodes is resolved by means of electric buffer. This paper develops a spanning tree construction algorithm, aiming at balancing input loads among output ports at each ingress node. Further a wavelength assignment algorithm is proposed, which is based on the amount of traffic lost at ingress nodes. We show that the proposed scheme can decrease the burst loss probability drastically, even if traffic intensities at ingress nodes are different.