Mass++ is a visualization/analysis tool for mass spectrometry, developed by Eisai Co.Ltd and Shimadzu Corp. under the CREST project by JST (Japan Science and Technology Agency) and FIRST program by CAO (the Cabinet Office of Japan) throughout the last nine years. The most characteristic feature of Mass++ is its plug-in structure, which enables users to customize it, namely add/remove functions, and supports to develop original functions without modifying the original Mass++ source code. Thus Mass++ has been distributed as freeware but without source codes. However, the open-source style is strongly favored especially for fixing bugs in existing plug-ins and ensuring the reproducibility and transparency in academic projects. In order to meet these demands, the source codes of Mass++ (except for some plug-ins containing software patents) was released under the 3-clause BSD license from March 20, 2015. The additional development and maintenance of Mass++ will be operated by the open-source community members, anchored by the newly-organized “Mass++ Users Conference.” In this presentation, the current status of open-sourced Mass++ functions, newly-developed functions, and the open source software maintenance/development process in the user community will be introduced.