Apart from the visible matter we know, the universe is made up of dark matter: invisible, unknown matter, the existence of which could not be demonstrated yet. All over the world countries and research institutions are conducting studies to detect dark matter with different methods and devices. In Japan, the University of Tokyo is leading the XMASS project. The XMASS detector is a device for conducting experiments to detect dark matter using liquid Xenon and photomultiplier tubes. Under the guidance of researchers from the University of Tokyo, MTC contributed to the project designing and manufacturing the core part of the XMASS detector.