We have developed a novel cryostat in which glassy states of molecular substances are formed by deposition of their vapor on a cold substrate. The glassy samples are subjected to in situ inelastic neutron scattering experiments. By the use of this cryostat, the glassy state of propylene (CH2=CHCH3) prepared at 20 K was examined on an inverted geometry time-of-flight neutron spectrometer in the energy range below 10 meV. The measurement was performed also on the glassy sample annealed at the glass transition temperature (55 K). A broad excitation peak (boson peak) was found at around 3 meV in the S(2 theta,omega) spectra of the as-deposited and annealed samples. The absolute densities of vibrational states G(omega) were derived by combining the neutron spectra and the heat capacity data of a similar compound l-butene (CH2=CHCH2CH3). The number of vibrational modes associated with the low-energy excitation was 1.64 per molecule for the as-deposited sample and 1.34 for the. annealed sample. The boson peak energy depends on the molecular weight in a systematic way for the five molecular glasses (propylene, 1-butene, 3-methylpentane, ethylbenzene, ortho-terphenyl) studied so far; the larger the molecular weight, the smaller the boson peak energy. This indicates that the low energy excitations of molecular glasses are associated with the overall motions, probably rotational one of the molecules. A model for the low energy excitations of molecular glasses is proposed based on the results obtained so far.