In a blow-molding apparatus, a cooling device and a heating device are each driven by being supplied with electricity of a predetermined voltage from a supply power source. The blow-molding apparatus comprises: a monitoring device configured to constantly monitor fluctuation of the predetermined voltage; and an output automatic control mechanism configured to, in a case where the predetermined voltage monitored by the monitoring device fluctuates beyond a normal range, automatically fluctuate an output of at least one of the heating device and the cooling device to keep the output in a certain range, thereby adjusting at least one of the preform temperature and the ambient temperature to fall within a normal temperature range.