  We investigate electromagnetic properties of a topological crystalline
insulator (TCI) thin film under external electromagnetic fields. The TCI thin
film is a topological insulator indexed by the mirror-Chern number. It is
demonstrated that the gap closes together with the emergence of a pair of
gapless cones carrying opposite chirarities by applying in-plane magnetic
field. A pair of gapless points have opposite vortex numbers. This is a
reminiscence of a pair of Weyl cones in 3D Weyl semimetal. We thus present an a
magnetic-field induced semimetal-semiconductor transition in 2D material. This
is a giant-magnetoresistance, where resistivity is controlled by magnetic
field. Perpendicular electric field is found to shift the gapless points and
also renormalize the Fermi velocity in the direction of the in-plane magnetic
field.
