This study established a method for preparing mesoporous silica (MS) with various pore structures from monomodal to bimodal by changing the starting materials ratio with the same method and materials. We examined the effect of the pore structure of the resulting samples on CO$_2$ adsorption properties after modification using various amine species with two modification methods (chemical or physical). MS samples were prepared by the co-condensation of two different silica precursors of tetraethylorthosilicate and polymethylhydrosiloxane (PMHS) using a templating agent of triblock copolymer, Pluronic P123. The results show that we succeeded in preparing three types of MS with different porestructures (one in monomodal and two in bimodal) by changing only the amount of PMHS. The results of CO$_2$ adsorption experiments indicated that although the amine-modified amount was highest for physical modification, because of severepore plugging, their CO$_2$ adsorption was not significant compared to chemically modified samples, which showed higher saturated CO$_2$ adsorption amount of 2.1 mmol g$^{-1}$ with very high amine utilization efficiency. In terms of pore structure,although the conventional monomodal MS, MCM-48, showed the highest saturated CO$_2$ adsorption amount due to high specific surface area, the plugging of the pores interferes with CO$_2$ diffusion. On the other hand, the larger pores in the MS samples prepared in this study both in monomodal and bimodal pore structure contributed to faster CO$_2$ diffusion.