This paper presents theoretical foundations for quantitative easing (QE). Since the late 2000s, with no room for lowering policy interest rates, central banks in the major advanced economies have adopted various unconventional monetary policies. QE is one of those unconventional policies and has so far achieved visible results in practice. However, our theoretical understanding of how QE achieves these results remains incomplete. The purpose of this paper is to introduce an inflation-sensitive money provision rule and show theoretically how QE helps an economy escape from a liquidity trap.