In Japan, there has been growing debate about the competitiveness between "postal savings certificates" (PSCs) offered by the Ministry of Posts and Communications and time deposits offered by private banks. Since PSCs have an "American put-option" characteristic, the "option premium" implied in PSCs should be taken into account explicitly. Since the interest rates have been regulated, we must devise a new option pricing theory based on imperfect arbitrage under such assumptions as depositor's risk neutrality and Markov property in the interest rates movements. Empirical analyses show that the advantage of PSCs over time deposits offered by private banks increases as depositor's holding period becomes longer and the current interest rates become higher.