A bulletin board system (BBS) equipped with Key-Graph is proposed for supporting online chance discovery process. While conventional meeting for chance discovery with KeyGraph requires participants to meet in a meeting room, the paper aims to expand the discussion space over the Internet. In order to support online chance discovery process, the BBS has functions for assisting a user in writing scenarios with visual annotation on the KeyGraph. The system also has a function for retrieving similar scenarios. The experimental result with test subjects shows the effectiveness of the BBS for online chance discovery process.