  We study entanglement entropy of excited states in two dimensional conformal
field theories (CFTs). Especially we consider excited states obtained by acting
primary operators on a vacuum. We show that under its time evolution,
entanglement entropy increases by a finite constant when the causality
condition is satisfied. Moreover, in rational CFTs, we prove that this
increased amount of (both Renyi and von-Neumann) entanglement entropy always
coincides with the log of quantum dimension of the primary operator.
