A system for collecting carbon dioxide in flue gas includes a stack that discharges flue gas discharged from an industrial facility to outside, a blower that is installed at the downstream side of the stack and draws the flue gas therein, a carbon-dioxide collecting device that collects carbon dioxide in the flue gas drawn in by the blower, and a gas flow sensor arranged near an exit side within the stack. A drawing amount of the flue gas by the blower to the carbon-dioxide collecting device is increased until an flow rate of the flue gas from the stack becomes zero in the gas flow sensor, and when the discharged amount of flue gas from the stack becomes zero, drawing in any more than that amount is stopped, and the carbon dioxide in the flue gas is collected while the flue gas is drawn in by a substantially constant amount.