Lithium disilicate (LS2) has been a crucial parent composition for glass-ceramics since the 1950s because of its excellent chemical and physical durability. In addition, a wide range of electrical properties can be obtained by changing the composition and crystallinity. Bandgap energy is one of the critical electrical properties for designing new lithium silicate-based materials. In this study, the bandgap energy of a synthesized LS2 crystal is evaluated using electron energy-loss spectroscopy and X-ray photoelectron spectroscopy. These two techniques unambiguously establish that the bandgap energy of LS2 is 7.7-7.8 eV, which is in the vacuum ultraviolet region. This confirms the insulating nature of the LS2 crystal.