The Multicolor Simultaneous Camera for studying Atmospheres of Transiting exoplanets (MuSCAT) is an optical three-band (g'2-, r'2- and z_s,2_-band) imager that was recently developed for the 188cm telescope at Okayama Astrophysical Observatory with the aim of validating and characterizing transiting planets. In a pilot observation with MuSCAT we observed a primary transit of HAT-P-14b, a high-surface gravity (g_p_=38ms^-2^) hot Jupiter around a bright (V=10) F-type star. From a 2.9hr observation we achieved the five-minute binned photometric precisions of 0.028%, 0.022%, and 0.024% in the g'2, r'2, and z_s,2_ bands, respectively, which provided the highest-quality photometric data for this planet. Combining these results with those of previous observations, we search for variations of transit timing and duration over five years as well as variations of planet-star radius ratio (Rp/Rs) with wavelengths, but can find no considerable variation in any parameters. On the other hand, using the transit-subtracted light curves we simulate the achievable measurement error of Rp/Rs with MuSCAT for various planetary sizes, assuming three types of host stars: HAT-P-14, the nearby K-dwarf HAT-P-11, and the nearby M-dwarf GJ1214. Comparing our results with the expected atmospheric scale heights, we find that MuSCAT is capable of probing the atmospheres of planets as small as a sub-Jupiter (Rp~6R_{earth}) around HAT-P-14 in all bands, a Neptune (~4R{earth}) around HAT-P-11 in all bands, and a super-Earth (~2.5R{earth}) around GJ1214 in r'_2 and z_s,2_ bands. These results promise that MuSCAT will produce fruitful scientific outcomes in the K2 and TESS era.