We report on tensor renormalization group calculations of entanglement entropy
in one-dimensional quantum systems.
The reduced density matrix of a Gibbs state can be represented as a $1 + 1$-dimensional
tensor network, which is analogous to the tensor network representation of the partition function.
The HOTRG method is used to approximate the reduced density matrix for arbitrary subsystem sizes,
from which we obtain the entanglement entropy.
We test our method in the quantum Ising model and obtain the entanglement entropy of the ground state
by taking the size of time direction to infinity.
The central charge $c$ is obtained as $c = 0.49997(8)$ for a bond dimension $D_\text{cut}=96$,
which agrees with the theoretical value $c=1/2$ within the error.