  Neutron elastic scattering experiments have been performed on the spin gap
system TlCuCl$_3$ in magnetic fields parallel to the $b$-axis. The magnetic
Bragg peaks which indicate the field-induced N\'{e}el ordering were observed
for magnetic field higher than the gap field $H_{\rm g}\approx 5.5$ T at $Q=(h,
0, l)$ with odd $l$ in the $a^*-c^*$ plane. The spin structure in the ordered
phase was determined. The temperature and field dependence of the Bragg peak
intensities and the phase boundary obtained were discussed in connection with a
recent theory which describes the field-induced N\'{e}el ordering as a
Bose-Einstein condensation of magnons.
