We measure a strong excess in the galaxy number density around PG1630+377, an extremely massive (MBH~109.7M{sun}) quasar at z=1.475, using near-infrared narrowband imaging. We identify 79 narrow H-band excess objects in a 525arcmin^2^ area including the vicinity and surroundings of the quasar. These sources are likely H{alpha} line emitting, star-forming galaxies at z~1.47. We detect a {delta}=6.6{+/-}2.7 overdensity of narrow H-band excess objects located at a projected distance ~2.1Mpc northeast of the quasar, which is the densest region in the target area. The overdensity is present in BzK color-selected galaxies, while a previously reported overdensity in the immediate vicinity of PG1630+377 is not, and yet appears as a group-like structure. These megaparsec-scale environments are estimated to merge into a ~1014.7M{sun} cluster at present. Our results support the view that extremely massive black holes form and grow in group-scale environments and later incorporate into a galaxy cluster.