Standard big bang nucleosynthesis (BBN) has the so called lithium problem, which refers to the discrepancy in primordial lithium abundance between the theory and the inferred observations of metal poor stars. We will first survey possible scenarios and various previous studies for the solution, and then explain that the discrepancy can be removed if we introduce a small amount of transient non-thermal particles that are time-dependently allowed to contribute only for a specific period of the BBN evolution. We will discuss the origin of such non-thermal particles, which is yet to be identified.