To provide a volume-reducible paper cup, which can be manufactured through a conventional paper cup manufacturing process and reduced in volume when scrapped after being used up.

The volume-reducible paper cup is mainly made of paper and has a trunk member and a bottom member, which are joined at a joint. In manufacturing the paper cup, a trunk member blank is rolled cylindrically to form the trunk member of an inverse truncated cone, while the outer circumferential edge of a bottom member blank is folded downward to form the bottom member having a bottom surface with a bent part on the outer circumference thereof. The bent part of the bottom member is sandwiched between a folded part formed by folding back the lower end of the trunk member and the trunk of the trunk member. In this state, the trunk member and the bottom member are joined temporarily at the joining strength that allows the bottom member to separate from the trunk member when the center of the bottom outer surface of the bottom member is pushed inward.