The present invention provides a manufacturing method of a battery, comprising: a preparation process of arranging a battery main body part (23) in a bag body which is a square shape when planarized, and preparing a temporal seal member (30) having a remaining first side part (34) as an opened opening (24) while three side parts of four side parts around the battery main body part of the bag body are sealed; an injection process of arranging the temporal seal member in a state of facing the opening (24) upward and opening the same, and injecting electrolyte into the battery main body part (23) by arranging an upper end of an electrolyte injection nozzle (40) immediately above the opening (24) or inserting the upper end thereof through the opening (24); a primary sealing process of forming a first sealing part by sealing the opening (24) with heat by using a first heat seal while deaerating the temporal seal member after the injection process; and a secondary sealing process of pressing the first sealing part with heat by using a second heat seal after the deaeration. According to the manufacturing method, enough sealing strength can be secured, thereby preventing each heat seal from being delaminated and manufacturing a battery having excellent productivity.