We propose the notion of spin-selective Kondo insulator, which provides a fundamental mechanism to describe the ferromagnetic phase of the Kondo lattice model with antiferromagnetic coupling. This unveils a remarkable feature of the ferromagnetic metallic phase: the majority-spin conduction electrons show metallic- while the minority-spin electrons show insulating-behavior. The resulting Kondo gap in the minority spin sector, which is due to the cooperation of ferromagnetism and partial Kondo screening, evidences a dynamically-induced commensurability for a combination of minority-spin electrons and parts of localized spins. Furthermore, this mechanism predicts a nontrivial relation between the macroscopic quantities such as electron magnetization, spin polarization and electron filling.