This study aims to suggest an improved experimental method to reveal the ability of indoor plants to reduce $CO_2$ concentrations, as well as to display the individual $CO_2$ reduction characteristics of various indoor plants in accordance with this improved method. In previous studies, experiments were conducted under the condition in which the $CO_2$ concentration in the experimental chamber is set only once to a high initial level of 1,000 ppm. However, in real conditions, $CO_2$ concentration gradually increases in a room after the occupants enter. Hence, the existing experimental method can be improved in view of "light saturation and $CO_2$ compensation". Accordingly, in this study, the $CO_2$ reduction characteristics of indoor plants under 2 conditions used in the existing method of measurement (Case 1) and the condition in the new method, which considers that $CO_2$ concentration gradually increases through the respiration of experimental animals (Case 2)-were measured and compared against each other. For all plant samples, the level of $CO_2$ reduction was higher in Case 2 than in Case 1, and the rate of $CO_2$ reduction increases with time. The inflection point of $CO_2$ concentration appeared at leaf areas of $9,000cm^2$ in peace lily and areca palm, and $6,000cm^2$ in weeping fig.