To provide a free residual chlorine concentration calculation method, a free residual chlorine concentration calculation system, and a space cleaning system with its bacterial proliferation suppression function, which detects free residual chlorine concentration in an aqueous solution by a method capable of easily and automatically adjusting the concentration and confirms optimum free residual chlorine concentration for suppressing bacterial proliferation.SOLUTION: A free residual chlorine concentration calculation method includes chlorine component addition means (sodium hypochlorite addition means 2) for adding a chlorine component (sodium hypochlorite) into an aqueous solution R and electric conductivity measurement means 3 for measuring an electric conductivity of the aqueous solution R before and after adding the chlorine component (sodium hypochlorite), and confirms by calculating free residual chlorine concentration of the aqueous solution R from an increment of the electric conductivity measured by the electric conductivity measurement means 3.SELECTED DRAWING: Figure 3【課題】水溶液中の遊離残留塩素濃度を、簡易的且つ濃度調整を自動化可能な方法で検出して、菌の増殖を抑制するために最適な遊離残留塩素濃度の確認を行う遊離残留塩素濃度算出方法、遊離残留塩素濃度算出システム、及び、その菌増殖抑制機能付き空間洗浄システムを提供する。【解決手段】水溶液Ｒ中へ塩素成分（次亜塩素酸ナトリウム）を添加する塩素成分添加手段（次亜塩素酸ナトリウム添加手段２）と、塩素成分（次亜塩素酸ナトリウム）添加前後の水溶液Ｒの電気伝導率を計測する電気伝導率計測手段３と、を備え、電気伝導率計測手段３で計測した電気伝導率の増分から水溶液Ｒの遊離残留塩素濃度を算出して確認する。【選択図】図３
