A determination system for determining whether metal lithium is precipitated in a lithium ion secondary battery includes: a discharging unit that causes the lithium ion secondary battery to perform constant current discharge until a voltage of the lithium ion secondary battery becomes a voltage corresponding to a predetermined low state of charge; a natural increase acquisition unit that acquires a natural increase in voltage of the lithium ion secondary battery after the constant current discharge is terminated; and a precipitation determining unit the compares the acquired natural increase with a predetermined threshold, that determines that the metal lithium is not precipitated when the natural increase is larger than or equal to the threshold, and that determines that the metal lithium is precipitated when the natural increase is smaller than the threshold.