Most stars are born as clusters in Giant Molecular Clouds (hereafter GMCs), and therefore the understanding of the evolution of GMCs in a galaxy is one of the key issues to investigate the evolution of the galaxy. The recent state-of-the-art radio telescopes have been enabling us to reveal the distribution of GMCs extensively in the Galaxy as well as in the nearby galaxies, and the physical properties and the evolution of the GMCs leading to cluster formations are actively being investigated. Here we present studies of spatially resolved GMCs in the Galaxy and in the Magellanic Clouds(LMC/SMC), aiming at determining the origins of the observed turbulence and assessing the role of gas interaction in triggering star formation by the observations spanning a range of scales and environmental conditions.

We have carried out ALMA observations toward ~10 GMCs located across the LMC by using ALMA mainly in the CO isotopologue lines of J=1-0 and 2-1 and continuum bands. The typical angular resolution is 1~3 arcsecs; 1 arcsec corresponds to 0.24pc at the distance of the LMC. These clouds have different evolutionary stages spanning a wide range of star formation activity. The observations revealed the complex nature of the molecular gas in the LMC; full of filaments and clumps; we also quantify their density structure and velocity-size correlations. The comparison with the galactic GMCs indicates the similarity of multiple filaments entangled toward the region where high-mass stars are forming. Further high-resolution observations have been done toward N159, which is the most intense and concentrated molecular cloud as shown by the brightest CO J=3-2 source in the LMC. The spatial resolution is 0.06pc, which is high enough to spatially resolve high-density cores and narrow filaments. The observations revealed 0.1pc width filaments entangled as well as high density cores as the possible site of proto cluster formation. We have also carried out 0.3pc resolution observations of N83C, a high mass star-forming region in the SMC. The radiative transfer analysis suggests that the kinetic temperature is ~40K and the density is a few x 10^4cm^-3, which is consistent with the virial analysis. This high-density, implying a lack of lower-density envelope, and the high temperature indicates that UV radiation deeply penetrates into the clump and CO molecule is heavily photodissociated in the low-metalicity environment.