PhD at NAOJ/SOKENDAI (The Graduate University for Advanced Studies), 2017 Worked at Tohoku University and the University of Groningen as a postdoc Moved to Kavli IPMU in October, 2021The discovery in deep near-infrared surveys of a population of massive quiescent galaxies at z > 3 has given rise to the question of how they came to be quenched so early in the history of the Universe. Follow-up observations at mm/sub-mm wavelengths of such quiescent galaxies are crucial to confirm the presence of the dust-obscured star-formation and give a constraint on their molecular gas contents. Measuring their molecular gas properties allows us to distinguish between physical processes where they stop forming stars due to a lack of fuel versus those where star-formation efficiency is reduced and the gas is retained. We conducted ALMA observations of four quiescent galaxies at z = 3.5-4.0 found by the Fourstar Galaxy Evolution Survey (ZFOURGE) and a serendipitous optical-dark galaxy at z = 3.71. The Band-7 observations confirm the passive nature of the four quiescent galaxies. We also find that all the targets have gas mass fractions of < 20% with the Band-3 data targeting the atomic carbon line. In this talk, I will introduce previous works about quiescent galaxies at z > 3 done by the ZFOURGE team. Then I will show the latest results on their gas properties with ALMA and discuss a possible quenching mechanism of massive galaxies at z > 3.5.