In the Battle of Sekigahara in 1600 his son Kazumasa IKOMA was on the part of the East squad while Chikamasa, staying in his territory, was on the side of the West squad and delegated his vassal to join the siege of Tanabe-jo Castle in Tango Province.
1600年の関ヶ原の戦いでは、息子の生駒一正は東軍に与し、親正は在国していたが西軍に与して丹後国田辺城(丹後国)攻めに家臣を代理として派遣した。