  We observe the magnetic oscillation of electric conductance in the
two-dimensional InAs/GaSb quantum spin Hall insulator. Its insulating bulk
origin is unambiguously demonstrated by the antiphase oscillations of the
conductance and the resistance. Characteristically, the in-gap oscillation
frequency is higher than the Shubnikov-de Haas oscillation close to the
conduction band edge in the metallic regime. The temperature dependence shows
both thermal activation and smearing effects, which cannot be described by the
Lifshitz-Kosevich theory. A two-band Bernevig-Hughes-Zhang model with a large
quasiparticle self-energy in the insulating regime is proposed to capture the
main properties of the in-gap oscillations.
