A ballast water treatment device comprising: a ballast tank; a main pipe which guides ballast water to the ballast tank; a ballast pump arranged in the main pipe; a circulation pipe which returns ballast water stored in the ballast tank to the main pipe; a sterilizing component-introducing unit for introducing a sterilizing component to a downstream side of a part of the main pipe, the part being connected with the circulation pipe, one end of the circulation pipe being connected to the ballast tank and the other end of the circulation pipe being connected to a first connection part located upstream of the ballast pump and the sterilizing component-introducing unit in the main pipe; a concentration measurement unit for measuring a sterilizing component concentration of ballast water stored in the ballast tank, a filter arranged downstream of the ballast pump in the main pipe, the sterilizing component-introducing unit being arranged downstream of the filter in the main pipe; a detour pipe detouring the filter and including a first end and a second end, the first end being connected to a second connection part of the main pipe located upstream of the filter and downstream of the ballast pump, and the second end being connected to a third connection part of the main pipe located downstream of the filter and upstream of the sterilizing component-introducing unit; a mixer arranged downstream of the sterilizing component-introducing unit in the main pipe; a first switching valve arranged in the circulation pipe to adjust an amount of circulating ballast water to flow through the circulation pipe; a second switching valve arranged upstream of the first connection part to adjust an amount of seawater to be taken into the main pipe; a third switching valve arranged downstream of the second connection part and upstream of the third connection part to make adjustment for allowing seawater to flow to the filter or ballast water to flow to the detour pipe; and a control unit configured to open the first switching valve and close the second and third switching valves (71B, 71C) and activate the ballast pump so that the ballast water flowing through the main pipe and returned from the ballast tank via the circulation pipe returns to the ballast tank via the detour pipe, wherein an amount of a sterilizing component in the ballast water in the ballast tank is adjusted by introducing the sterilizing component from the sterilizing component-introducing unit into the main pipe when the ballast water in the ballast tank flows through the main pipe via the circulation pipe and is returned to the ballast tank via the detour pipe. a ballast tank; a main pipe which guides ballast water to the ballast tank; a ballast pump arranged in the main pipe; a circulation pipe which returns ballast water stored in the ballast tank to the main pipe; a sterilizing component-introducing unit for introducing a sterilizing component to a downstream side of a part of the main pipe, the part being connected with the circulation pipe, one end of the circulation pipe being connected to the ballast tank and the other end of the circulation pipe being connected to a first connection part located upstream of the ballast pump and the sterilizing component-introducing unit in the main pipe; a concentration measurement unit for measuring a sterilizing component concentration of ballast water stored in the ballast tank, a filter arranged downstream of the ballast pump in the main pipe, the sterilizing component-introducing unit being arranged downstream of the filter in the main pipe; a detour pipe detouring the filter and including a first end and a second end, the first end being connected to a second connection part of the main pipe located upstream of the filter and downstream of the ballast pump, and the second end being connected to a third connection part of the main pipe located downstream of the filter and upstream of the sterilizing component-introducing unit; a mixer arranged downstream of the sterilizing component-introducing unit in the main pipe; a first switching valve arranged in the circulation pipe to adjust an amount of circulating ballast water to flow through the circulation pipe; a second switching valve arranged upstream of the first connection part to adjust an amount of seawater to be taken into the main pipe; a third switching valve arranged downstream of the second connection part and upstream of the third connection part to make adjustment for allowing seawater to flow to the filter or ballast water to flow to the detour pipe; and a control unit configured to open the first switching valve and close the second and third switching valves (71B, 71C) and activate the ballast pump so that the ballast water flowing through the main pipe and returned from the ballast tank via the circulation pipe returns to the ballast tank via the detour pipe, wherein an amount of a sterilizing component in the ballast water in the ballast tank is adjusted by introducing the sterilizing component from the sterilizing component-introducing unit into the main pipe when the ballast water in the ballast tank flows through the main pipe via the circulation pipe and is returned to the ballast tank via the detour pipe.