CO(J=1-0) line is usually observed to trace cold molecular gas since its luminosity is proportional to molecular gas mass in general. However, some studies measured molecular gas mass by CO(J=2-1) line with the assumed intensity ratio of CO(J=2-1)/CO(J=1-0) (R_2/1) as a constant. To verify the assumption of the constant R_2/1, we measured R_2/1 in 29 nearby galaxies observed with the Nobeyama 45-m telescope and the IRAM 30-m telescope. The median of R_2/1 combined all samples is 0.58, which is smaller than a previous result of ~ 0.7. We compared the Kennicutt-Schmidt (K-S) relation with molecular gas surface density measured by CO(J=1-0), and CO(J=2-1) assuming the constant R_2/1 of 0.7. We found that the index of the K-S relation for most of galaxies is smaller when the relation is derived by CO(J=2-1) and the constant R_2/1. R_2/1 shows moderate positive correlation between star-formation efficiency. It suggests R_2/1 is related with global star-formation. We conclude CO(J=2-1) line cannot be a tracer of accurate molecular gas mass with the assumed R_2/1 as a constant.