  We present wide-field JHKs-band photometric observations of the three compact
HII regions G48.9-0.3, G49.0-0.3, and G49.2-0.3 in the active star-forming
region W51B. The star clusters inside the three compact HII regions show the
excess number of stars in the J-Ks histograms compared with reference fields.
While the mean color excess ratio E(J-H)/E(H-Ks) of the three compact HII
regions are similar to ~ 2.07, the visual extinctions toward them are somewhat
different: ~ 17 mag for G48.9-0.3 and G49.0-0.3; ~ 23 mag for G49.2-0.3. Based
on their sizes and brightnesses, we suggest that the age of each compact HII
region is =< 2 Myr. The inferred total stellar mass, ~ 1.4 x 10^4 M_sun, of
W51B makes it one of the most active star forming regions in the Galaxy with
the star formation efficiency of ~ 10 %.
