As seen in packet analysis of TCP/IP offload engine and stream data processing for video/audio data, it is necessary to extract a particular data field from bulk data, where we can use a field-data extractor. Particularly, an (M, N)-field-data extractor reads out any consecutive N bytes from an M-byte register by connecting its input/output using multiplexers. However, the number of required multiplexers increases too much as the input/output byte lengths increase. How to reduce the number of its required multiplexers is a major challenge. In this paper, we propose an efficient multiplexer network synthesis method for an (M, N)-field-data extractor. Our method is based on inserting an (N + B-1)-byte virtual intermediate register into a multiplexer network and partitioning it into an upper network and a lower network. Our method theoretically reduces the number of required multiplexers without increasing the multiplexer network depth. We also propose how to determine the size of the virtual intermediate register that minimizes the number of required multiplexers. Experimental results show that our method reduces the required number of gates to implement a field-data extractor by up to 92% compared with the one using a naive multiplexer network.