  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 of
CO(2-1)/CO(1-0), $R_{21}$, on GMC scale is important to accurately estimate the
mass of the GMCs. We thus measured the $R_{21}$ in the strongly barred galaxy
NGC1300, where star formation activity strongly depends on galactic structure,
on $\sim 100$ pc scale. CO images were obtained from ALMA and Nobeyama 45-m
telescope. The resultant typical $R_{21}$ in NGC1300 is $0.57 \pm 0.06$. We
find environmental variations in $R_{21}$; it is the highest in the bar-end
region ($0.72 \pm 0.08$), followed by arm ($0.60 \pm 0.07$) and bar regions
($0.50 \pm 0.06$). GMCs with H$\alpha$ emission show a systematically higher
ratio ($0.67 \pm 0.07$) than those without H$\alpha$ ($0.47 \pm 0.05$). In the
bar region, where massive star formation is suppressed, H$\alpha$ emission is
not associated with most GMCs, resulting in the lowest $R_{21}$. These results
raise a possibility that properties of GMCs derived from CO(2-1) observations
with the assumption of a constant $R_{21}$ are different from those derived
from CO(1-0) observations. Furthermore, we find the $R_{21}$ measured on kpc
scale tends to be lower than that of the GMCs probably due to the presence of
an extended diffuse molecular gas in NGC1300.
