One's biological (real) father — used to distinguish from an adoptive or stepfather.
出生届には実父の名前を書く必要がある。
You need to write the biological father's name on the birth registration.
彼は実父と再会して感動の涙を流した。
He met his biological father again and cried tears of emotion.
実父の家系について詳しく調べたいと言った。
He said he wanted to investigate his biological father's family history in detail.