A shared disks (SD) cluster couples multiple computing nodes for high performance transaction processing, and all nodes share a common database at the disk level. To reduce the number of disk access, the node may cache both data pages and index pages in its local memory buffer. In general, index pages are accessed more often and thus cached at more nodes than their corresponding data pages. Furthermore, tree-based indexes such as B+ trees require complicated operations, e.g., root-to-leaf traversal and structure modification operation due to a page split or a page concatenation. This means that it is strongly required to devise a dedicated cache coherency scheme for index pages that takes advantage of the semantics of their access. In this paper, we propose a new cache coherency scheme for B+ tree indexes in the SD cluster. The proposed scheme can reduce the message traffic between nodes and the number of tree re-traversals. Using a simulation model of the SD cluster, we show that the proposed scheme exhibits substantial performance improvement over the previous schemes.