  Evidence of a heavy neutrino, $\nu_H$, in the $K^+\to\mu^+\nu_H$ decays was
sought using the E949 experimental data with an exposure of $1.70\times
10^{12}$ stopped kaons. With the major background from the radiative
$K^+\to\mu^+\nu_\mu\gamma$ decay understood and suppressed, upper limits (90%
C.L.) on the neutrino mixing matrix element between muon and heavy neutrino,
$|U_{\mu H}|^2$, were set at the level of $10^{-7}$ to $10^{-9}$ for the heavy
neutrino mass region 175 to 300 MeV/$c^2$.
