This paper provides an empirical investigation of monetary policy in Japan in the zero interest rate environment that has held sway since 1999. In particular, we focus on the effects of the zero interest rate commitment and of quantitative monetary easing on medium- to long-term interest rates in Japan. In the study we apply a version of the macro-finance approach, involving a combination of estimation of a structural macro-model and calibration of time-variant parameters to the yield curve observed in the market. This enables us to decompose interest rates into expectations and risk premium components and simultaneously to extract the market's perception of the Bank of Japan's (BOJ's) willingness to carry on its zero interest rate policy. In the analysis we make clear the counterfactual policy that would have been practiced in the absence of the actual policies followed by the BOJ since 1999. From this analysis, we tentatively conclude that the BOJ's monetary policy since 1999 has functioned mainly through the zero interest rate commitment, which has led to declines in medium- to long-term interest rates. We also find some evidence that, up until the end of 2003, raising the reserve target may have been perceived as a signal indicating the BOJ's accommodative policy stance although the size of the effect is not large. The portfolio rebalancing effect - either by the BOJ's supplying ample liquidity or by its purchases of long-term government bonds - has not been found to be significant.