The inhibition mechanism of a water-miscible ionic liquid, N-butyl-3-methypyridinium tetrafluoroborate ([BMPy][$BF_4$]), on the catalysis of horseradish peroxidase (HRP) was investigated. The $K_m$ value for the oxidation of guaiacol (2-methoxyphenol) with $H_2O_2$ catalyzed by HRP increased from 2.8 mM in 100% water to 12.6 mM in 25% (v/v) [BMPy][$BF_4$]. This increase of $K_m$ by the ionic liquid was elucidated to be caused by the strong stabilization of the ground state of guaiacol by the ionic liquid. On the contrary, the $k_{cat}$ value for the HRP-catalyzed reaction decreased from 13.8/sec in 100% water to 6.7/sec in 25% (v/v) [BMPy][$BF_4$]. Such decrease of $k_{cat}$ value of HRP catalysis by the increasing content of [BMPy][$BF_4$] was described using the noncompetitive inhibition of the enzyme by the ionic liquid. The value of the inhibition constant of [BMPy][$BF_4$] was 1.48 M indicating that the ionic liquid exerts a weak noncompetitive inhibition effect on the HRP catalysis.