To explain the large isotope effect on the transition temperature in KH2PO4, the tunneling motion of the protons in hydrogen bonds is considered and a theory of the phase transition is developed under the assumption that the ferroelectric behavior of this substance is due to cooperation of the ionic displacements, the interactions between ions and protons and the tunneling motion of protons. The relation between the present theory and the Slater model or the Blinc model is discussed.