A carbon dioxide capturing system comprising: an absorption tower configured to bring a treatment target gas containing carbon dioxide into contact with an absorption liquid for absorbing the carbon dioxide, and to discharge the absorption liquid having absorbed the carbon dioxide and an absorption tower exhaust gas containing the treatment target gas from which the carbon dioxide has been removed; a regeneration tower configured to make the absorption liquid discharged from the absorption tower dissipate the carbon dioxide, and to discharge the absorption liquid having dissipated the carbon dioxide and a regeneration tower exhaust gas containing the carbon dioxide; and a treatment target gas line configured to introduce the treatment target gas into the absorption tower; the system further comprising: a first introduction module (23, 16) configured to introduce a first gas having a higher carbon dioxide concentration than the treatment target gas into the treatment target gas line; a second introduction module (25, 24, 32, 11) configured to introduce a second gas having a lower carbon dioxide concentration than the treatment target gas into the treatment target gas line; and wherein the system is configured to introduce the first gas into the treatment target gas line, when a carbon dioxide concentration of the treatment target gas is lower than a lower limit, and to introduce the second gas into the treatment target gas line, when the carbon dioxide concentration of the treatment target gas is higher than an upper limit; or the system is configured to introduce the first and second gases into the treatment target gas line, when a gas flow rate of the treatment target gas is lower than a lower limit. an absorption tower configured to bring a treatment target gas containing carbon dioxide into contact with an absorption liquid for absorbing the carbon dioxide, and to discharge the absorption liquid having absorbed the carbon dioxide and an absorption tower exhaust gas containing the treatment target gas from which the carbon dioxide has been removed; a regeneration tower configured to make the absorption liquid discharged from the absorption tower dissipate the carbon dioxide, and to discharge the absorption liquid having dissipated the carbon dioxide and a regeneration tower exhaust gas containing the carbon dioxide; and a treatment target gas line configured to introduce the treatment target gas into the absorption tower; the system further comprising: a first introduction module (23, 16) configured to introduce a first gas having a higher carbon dioxide concentration than the treatment target gas into the treatment target gas line; a second introduction module (25, 24, 32, 11) configured to introduce a second gas having a lower carbon dioxide concentration than the treatment target gas into the treatment target gas line; and wherein the system is configured to introduce the first gas into the treatment target gas line, when a carbon dioxide concentration of the treatment target gas is lower than a lower limit, and to introduce the second gas into the treatment target gas line, when the carbon dioxide concentration of the treatment target gas is higher than an upper limit; or the system is configured to introduce the first and second gases into the treatment target gas line, when a gas flow rate of the treatment target gas is lower than a lower limit. a first introduction module (23, 16) configured to introduce a first gas having a higher carbon dioxide concentration than the treatment target gas into the treatment target gas line; a second introduction module (25, 24, 32, 11) configured to introduce a second gas having a lower carbon dioxide concentration than the treatment target gas into the treatment target gas line; and wherein the system is configured to introduce the first gas into the treatment target gas line, when a carbon dioxide concentration of the treatment target gas is lower than a lower limit, and to introduce the second gas into the treatment target gas line, when the carbon dioxide concentration of the treatment target gas is higher than an upper limit; or the system is configured to introduce the first and second gases into the treatment target gas line, when a gas flow rate of the treatment target gas is lower than a lower limit.