This study was aimed at developing and evaluating a diffusive sampling method using a barium hydroxide solution as an absorbent for measuring carbon dioxide ($CO_2$) in ambient air. The collected $CO_2$ concentration was calculated by the change of conductivity resulted in the reaction of $CO_3{^{2-}}$ and $Ba^{2+}$ in aqueous solution. The sampling rate for the diffusive sampler was determined 0.218 mL/min, as obtained from the slope of the linear correlation between the $CO_2$ mass collected by the diffusive sampler and the time-weighted $CO_2$ concentration with the active sampling method. The unexposed blank sampler sealed in aluminium foil-polyethylene laminated packets has remained stable during at least one-month storage period. A good correlation was observed between the diffusive sampler and active sampler with a coefficient of determination of 0.956. This diffusive sampler would be suitable for the indoor $CO_2$ concentration monitoring.