A biological (real) parent—used to distinguish one's birth parent from adoptive or foster parents.
彼は子ども時代に養子に出され、成人してから実の親と再会した。
He was put up for adoption as a child and reunited with his biological parents as an adult.
実の親を探すために戸籍や古い記録を調べた。
I checked family registers and old records to search for my biological parents.
養子を育てる立場でも、実の親としての責任を感じることがある。
Even as an adoptive parent, one may feel responsibilities similar to those of a biological parent.