Yasutoyo was a younger son of [[Yamauchi Moritoyo]]. He assisted his elder brother [[Yamauchi Kazutoyo]] in quelling the difficult elements of [[Tosa province]] and was awarded the 20,000 ''[[koku]]'' branch domain of [[Nakamura han (Tosa)|Nakamura]]. After Kazutoyo died without an heir, Yasutoyo's son [[Yamauchi Tadayoshi]] succeeded Kazutoyo as lord of [[Tosa han]].
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Yasutoyo was a younger son of [[Yamauchi Moritoyo]]. He assisted his elder brother Kazutoyo in quelling the difficult elements of [[Tosa province]] and was awarded the district of [[Nakamura district|Nakamura]] and [[daimyo|daimyô]] status.