  Lattice regularized Schwinger model with a so-called $\theta$ term is studied
by using the Grassmann tensor renormalization group. We perform the Lee-Yang
and Fisher zero analyses in order to investigate the phase structure at
$\theta=\pi$. We find a first order phase transition at larger fermion mass.
Both of the Lee-Yang zero and Fisher zero analyses indicate that the critical
endpoint at which the first order phase transition terminates belongs to the
Ising universality class.
