A device for sealing a container door are configured for the sake of a container security in such a way that the device for sealing a container door is installed at a locking device of a container door so as to detect if the container door is forcibly opened or closed by an unauthorized person during a container transportation from a freight-loaded place to a freight-unloaded place, and a result of such a detection is wirelessly transmitted to a container security system. The device for sealing a container door includes a main body, a locking member, a fixing unit, a stopper unit, a seal-release authentication unit, a holding unit, a seal-sensing unit, a LED state display unit, a battery, and a control board.