  The recent observation [Oosawa et al. J. Phys. : Condens. Matter {\bf 11},
265 (1999)] of the field-induced N\'{e}el ordering in a spin-gap magnetic
compound TlCuCl$_3$ is interpreted as a Bose-Einstein Condensation of magnons.
A mean-field calculation based on this picture is shown to describe well the
temperature dependence of the magnetization. The present system opens a new
area for studying Bose-Einstein condensation of thermodynamically large number
of particles in a grand-canonical ensemble.
