  We investigate the electromagnetic cascade initiated by injection of very
high energy photons in the early Universe and calculate the cascade spectrum by
solving a set of Boltzmann equations numerically. In the calculation we take
account of Compton scattering off the background electrons and pair creation
off the background nucleons as well as photon-photon processes and inverse
Compton scattering. We also apply our cascade spectrum to the big bang
nucleosynthesis with photo-dissociation processes due to heavy unstable
particles and obtain the constraint on their lifetime and abundance.
