The Ishikawa clan of the Mikawa province were direct descendants of Minamoto no Yoshiie. Minamoto Yoshitoki, the son of Yoshiie, was to adopt the surname of "Ishikawa". The Ishikawa then moved to the Shimotsuke Province, and for a few generations became known as the "Oyama", which was borrowed from a local family. During the 15th century, the Ishikawa moved back to Mikawa, and once again became known as the Ishikawa. A notable member of the Ishikawa, Ishikawa Chikayasu, went on to serve under Matsudaira Chikatada. After this event, the Ishikawa for many years after became notable retainers under the Matsudaira/Tokugawa clan.

Reference

Nobunaga's Ambition

Japanese clans

石川氏