Delayed neutron emission is investigated theoretically. First, the strength functions for the allowed and first-forbidden β-decays of delayed neutron precursors are calculated by the gross theory of β-decay developed by Takahashi, Koyama and Yamada. It is shown that the strengths obtained by the sum rule considerations of the gross theory are less energy-dependent than those used by other investigators. Next, the absolute magnitudes of the delayed gular momentum. The general trends of the observed emission rates for many nuclides can be well explained. Furthermore, the energy spectra of delayed neutrons are calculated for several examples in consideration of the angular momentum. These results show that the gross theory of β-decay is favorably applicable to the study of delayed neutrons.