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The primordial lithium abundance inferred from observations of metal-poorstars is ~3 times smaller than the theoretical value in standard big bangnucleosynthesis (BBN) model. We assume a simple model including a sterileneutrino nu_H with mass of O(10) MeV which decays long after BBN. We theninvestigate cosmological effects of a sterile neutrino decay. We formulate theinjection spectrum of nonthermal photons induced by electrons and positronsgenerated at the nu_H decay, as a function of the nu_H mass and the photontemperature. We then consistently solve (1) the cosmic thermal history, (2)nonthermal nucleosynthesis induced by the nonthermal photons, (3) thebaryon-to-photon ratio eta, and (4) the effective neutrino number N_eff.Amounts of energy injection at the nu_H decay are constrained from limits onprimordial D and 7Li abundances, the N_eff value, and the cosmic microwavebackground energy spectrum. We find that 7Be is photodisintegrated and the Liproblem is partially solved for the lifetime 10^4-10^5 s and the mass >~ 14MeV. 7Be destruction by more than a factor of 3 is not possible because of anassociated D over-destruction. In the parameter region, the eta value isdecreased slightly, while the N_eff value is increased by a factor of <~ 1. Inthis study, errors in photodisintegration cross sections of 7Be(g, a)3He and7Li(g, a)3H that have propagated through literatures are corrected. It is thenfound that the new photodisintegration rates are 2.3 to 2.5 times smaller thanthe old rates, so that efficiencies of 7Be and 7Li photodisintegration aresignificantly smaller.

Effects of Long-lived 10 MeV Scale Sterile Neutrino on Primordial Elemental Abundances and Effective Neutrino Number