  It is emphasized that a rare decay $K^+ \rightarrow \pi^+ \nu \bar{\nu}$
becomes promising in a future search for a new particle, because the
theoretical treatment is well established and the value of the branching ratio
$Br(K^+ \rightarrow \pi^+ \nu \bar{\nu})$ is sensitive to a search for a new
particle with a TeV scale mass. As an example, according to a U(3) family gauge
boson model which predicts the lowest family gauge boson with a few TeV mass
$M_{11}$, the branching ratio $Br(K^+ \rightarrow \pi^+ \nu \bar{\nu})$ is
discussed. If we can obtain, in future, a slightly lower value $Br^{obs} \sim
0.9 \times 10^{-10}$ compared with the present observed value $Br^{obs}=(1.7\pm
1.1)\times 10^{-10}$, we can conclude $M_{11} \sim$ a few TeV.
