Systems and devices used for Industrial Control System (ICS) are basically designed for their exclusive use. In addition to those, generic products have been recently introduced to allow integration into wider usage. Some of them are connected to the Internet depending on circumstances. A number of Internet-reachable ICS devices without appropriate security measures have been observed, and these may be leveraged by threat actors for serious cyber security incidents. This paper explains the security threats surrounding ICS through attack scenarios for Internet-reachable ICS and its mitigation activities in Japan.