As for the 5′×4′(～11pc×9pc) region centered at W51 IRS1 the observations of the 3.4 mm continuum, CO (J=1—0) line and simultaneous NH3 (1,1), (2,2), (3,3), (4,4) inverse lines were made for studying the massive star formation region located in the main spiral arms of the Galaxy. In the directions of W51 IRS1, IRS2 and e1/e2 in 3.4 mm continuum, analyses of the line profiles show that the absorption lines of ammonia, which arise from the gas in front of the HII region, are red-shifted with respect to the emission lines, which arise from the surrounding cloud. Furthermore, a radiation transfer and statistical equilibrium calculation of ammonia molecules show that the densities increase by 3—10 times from the eastern border to the center. These points hint that the collapse is happening in the molecular cloud core obscured in optical wavelengths. The effects of the radiation fields from radio, infrared and UCHII sources is non-negligible on the excitation of various molecules (e.g. NH3) within the circle of radius 40″ centered at IRS1. The profiles of the CO J=1—0 line in the circle change from double peaks (～60, ～68 km*s-1) to triple peaks, i.e. the component ～53 km*s-1, which associates with UCHII, also appears in the spectra. There are indications that the circle of radius 40″ centered at IRS1 is a region of massive star forming activity.