An indexer robot includes two transport arms. The indexer robot simultaneously transports two unprocessed substrates from a cassette to a substrate transfer part in such a manner that each of the two transport arms holds a single unprocessed substrate. Also, the indexer robot simultaneously receives two processed substrates from the substrate transfer part and simultaneously transports the two processed substrate to a cassette in such a manner that each of the two transport arms holds a single processed substrate. The provision of three sending substrate rest parts and three returning substrate rest parts in the substrate transfer part enables the indexer robot to smoothly accomplish the simultaneous two-substrate transport, thereby reducing the time required for the transport of substrates in an entire substrate processing apparatus.