  More than 400 $K^{+}\to\pi^{+}\mu^+\mu^-$ events were observed in a rare
$K^+$ decay experiment at the AGS. Normalized to the
$K^{+}\to\pi^{+}\pi^+\pi^-$ decay, the branching ratio is determined to be
$(9.22 \pm 0.60 (stat) \pm 0.49 (syst))\times 10^{-8}$. This branching ratio
and the $\mu\mu$ mass spectrum is in very good agreement with the measurement
of the $K^{+}\to\pi^{+}e^+e^-$ decay, but deviates significantly from the
previous measurement.
