  Transverse muon polarization in the $K^+ \rightarrow \mu^+ \nu \gamma$ decay
is calculated in the model with scalar and pseudo scalar four-Fermi
interactions. Combined with a similar calculation in the $K^+ \rightarrow \mu^+
\nu \pi^0$ decay, a possible constraint on parameters in the three-Higgs model
is obtained assuming sensitivity of the up-coming KEK experiment. It is pointed
out that the predictions for the two polarizations are strongly correlated in
the three Higgs model.
