A separation apparatus comprising: a separator disposed in an oilfield, which separates water to be treated from crude oil produced in an oil well; an acquiring section being configured to acquire water to be treated containing oil field produced water that is produced associated with the crude oil produced in the oil well, the acquiring section acquiring the water to be treated from the separator; a separation section being configured to separate oil from the water to be treated, including: a storage section being configured to store flocculants for flocculating the oil contained in the water to be treated; a flocculating section being configured to form aggregates containing flocculated oil by stirring together the flocculants, magnetic materials, and the water to be treated; a collecting section being configured to collect and send the aggregates to the substance-to-be-removed discharging section by a collector having magnetism; and wherein the separation section includes a return section, the return section being configured to return at least a part of the treated water to the storage section in an anoxic state, wherein the storage section being configured to dissolve the flocculants in the treated water returned by the return section, a closed container being configured to maintain at least the separation section in a sealed state; an oxygen removing section being configured to remove oxygen in the separation section including: a supply pipe communicating with inside of the closed container and supplying the closed container with an anoxic oil-field-associated gas that is produced associated with the crude oil produced in the oil well, the supply pipe being connected to the separator, such that the anoxic oil-field-associated gas is supplied from the separator to the separation section of the closed container, and an exhaust pipe communicating to inside of the closed container, the exhaust pipe exhausting oxygen in the closed container; a substance-to-be-removed discharging section being configured to discharge the oil separated from the water to be treated; and a treated-water discharging section being configured to discharge the treated water remaining after the oil is separated from the water to be treated. a separator disposed in an oilfield, which separates water to be treated from crude oil produced in an oil well; an acquiring section being configured to acquire water to be treated containing oil field produced water that is produced associated with the crude oil produced in the oil well, the acquiring section acquiring the water to be treated from the separator; a separation section being configured to separate oil from the water to be treated, including: a storage section being configured to store flocculants for flocculating the oil contained in the water to be treated; a flocculating section being configured to form aggregates containing flocculated oil by stirring together the flocculants, magnetic materials, and the water to be treated; a collecting section being configured to collect and send the aggregates to the substance-to-be-removed discharging section by a collector having magnetism; and a storage section being configured to store flocculants for flocculating the oil contained in the water to be treated; a flocculating section being configured to form aggregates containing flocculated oil by stirring together the flocculants, magnetic materials, and the water to be treated; a collecting section being configured to collect and send the aggregates to the substance-to-be-removed discharging section by a collector having magnetism; and wherein the separation section includes a return section, the return section being configured to return at least a part of the treated water to the storage section in an anoxic state, wherein the storage section being configured to dissolve the flocculants in the treated water returned by the return section, a closed container being configured to maintain at least the separation section in a sealed state; an oxygen removing section being configured to remove oxygen in the separation section including: a supply pipe communicating with inside of the closed container and supplying the closed container with an anoxic oil-field-associated gas that is produced associated with the crude oil produced in the oil well, the supply pipe being connected to the separator, such that the anoxic oil-field-associated gas is supplied from the separator to the separation section of the closed container, and an exhaust pipe communicating to inside of the closed container, the exhaust pipe exhausting oxygen in the closed container; a supply pipe communicating with inside of the closed container and supplying the closed container with an anoxic oil-field-associated gas that is produced associated with the crude oil produced in the oil well, the supply pipe being connected to the separator, such that the anoxic oil-field-associated gas is supplied from the separator to the separation section of the closed container, and an exhaust pipe communicating to inside of the closed container, the exhaust pipe exhausting oxygen in the closed container; a substance-to-be-removed discharging section being configured to discharge the oil separated from the water to be treated; and a treated-water discharging section being configured to discharge the treated water remaining after the oil is separated from the water to be treated.