Evidence of a heavy neutrino, $ u_H$, in the $K^+tomu^+ u_H$ decays was sought using the E949 experimental data with an exposure of $1.70times 10^{12}$ stopped kaons. With the major background from the radiative $K^+tomu^+ u_mugamma$ 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/$c^2$.
