  We report on the energy spectrum of electrons in twisted bilayer graphene
(tBLG) obtained by the band-unfolding method in the tight-binding model. We
find the band-gap opening at particular points in the reciprocal space, that
elucidates the drastic reduction of the Fermi-level velocity with the tiny
twisted angles in tBLGs. We find that Moir\`e pattern caused by the twist of
the two graphene layers generates interactions among Dirac cones, otherwise
absent, and the resultant cone-cone interactions peculiar to each point in the
reciprocal space causes the energy gap and thus reduced the Fermi-level
velocity.
