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CO(2-1) emission is often used as a tracer of the giant molecular clouds(GMCs) as an alternative to CO(1-0) emission in recent years. Therefore,understanding the environmental dependence of the line ratio ofCO(2-1)/CO(1-0), $R_{21}$, on GMC scale is important to accurately estimate themass of the GMCs. We thus measured the $R_{21}$ in the strongly barred galaxyNGC1300, where star formation activity strongly depends on galactic structure,on $\sim 100$ pc scale. CO images were obtained from ALMA and Nobeyama 45-mtelescope. The resultant typical $R_{21}$ in NGC1300 is $0.57 \pm 0.06$. Wefind environmental variations in $R_{21}$; it is the highest in the bar-endregion ($0.72 \pm 0.08$), followed by arm ($0.60 \pm 0.07$) and bar regions($0.50 \pm 0.06$). GMCs with H$\alpha$ emission show a systematically higherratio ($0.67 \pm 0.07$) than those without H$\alpha$ ($0.47 \pm 0.05$). In thebar region, where massive star formation is suppressed, H$\alpha$ emission isnot associated with most GMCs, resulting in the lowest $R_{21}$. These resultsraise a possibility that properties of GMCs derived from CO(2-1) observationswith the assumption of a constant $R_{21}$ are different from those derivedfrom CO(1-0) observations. Furthermore, we find the $R_{21}$ measured on kpcscale tends to be lower than that of the GMCs probably due to the presence ofan extended diffuse molecular gas in NGC1300.

CO(2-1)/CO(1-0) line ratio on $\sim$100 parsec scale in the nearby barred galaxy NGC1300