  Terahertz radiation from the mesa structures of
Bi$_2$Sr$_2$CaCu$_2$O$_{8+\delta}$ is detected in samples with thin electrodes
$< 100$ nm. In samples with thick electrodes $\simeq$ 400 nm, neither
radiations nor voltage jumps in current-voltage characteristics are detected.
This suggests that the thin electrode helps excite the Josephson plasma
oscillation as a result of the poor heat flow through the electrode. The
shielding effect by the electrode is not essential. We consider that the local
temperature rise is the origin of the synchronization of the phase kink for
terahertz radiation.
