The physical properties of group and cluster galaxies are distinct from that of field galaxies.
Red and passive galaxies with a low star formation rate are dominant in dense environments.
Various environmental processes (e.g., tidal interaction and ram pressure stripping (RPS)) can play an important role
in changing galaxy properties (e.g., ISM and star formation) and galaxy evolution. Molecular gas is the direct fuel for star formation.
Therefore, in order to better understand a change of star formation in group and cluster galaxies,
it is essential to study how environmental processes affect the molecular gas of group and cluster galaxies.
Using high-resolution CO data (e.g., SMA and ALMA),
we find that the environmental processes can change molecular gas properties and star formation of group and cluster galaxies.
In particular, the asymmetric distribution of CO gas is found in our samples.
In this talk, I will show strong evidence that the molecular gas of Virgo spiral galaxies is disturbed by ram pressure.
I will also show preliminary results of our new CO study on group galaxies with the ALMA/ACA.