The present invention has a first depressurization step of depressurizing the inside of a liquid injecting chamber, an injection step of injecting, in the inside of the liquid injecting chamber, a liquid electrolyte into the battery through an opening portion formed at the top end of an outer case body of the battery, a second depressurization step of depressurizing the inside of a sealing chamber and a sealing step of sealing the opening portion in the inside of the sealing chamber. Vacuum attainment time ”T until the pressure of the inside of the liquid injecting chamber becomes at a predetermined vacuum degree is measured in the first depressurization step, and if this vacuum attainment time ”T exceeds first threshold ”Ts1, a depressurization rate is reduced and a depressurization time is extended.