Since Toshiharu AOYAMA, the lord of the Tanba-Kameyama Domain died without an heir, Tadatomo was adopted as his successor from a branch family which governed the Tango-Miyazu Domain.
享保15年（1730年）、先代藩主・青山俊春が嗣子なく没したため、分家である丹後宮津藩から養子に入る。