Lithium silicate has been studied mainly as a CO2 absorbent at high temperatures of around 773 K. It has recently been observed that lithium silicate also absorbs atmospheric CO2 at room temperature. In this study, lithium silicate absorbent was packed in an absorption column and the effectiveness of this column was confirmed. Based on this result, the effectiveness of lithium silicate in absorbing atmospheric CO2 at room temperature was evaluated. It was found that humidity promoted absorption. This mechanism was examined and ascribed to the relationship between the water content in lithium silicate and CO2 absorption. In addition, the effectiveness of lithium silicate absorbent was compared with that of a commercially available absorbent (soda lime) under conditions of controlled humidity, and the superiority of lithium silicate was confirmed.