An ejector including a nozzle having a flow path in which a motive fluid flowing from an upstream side undergoes pressure reduction and is made to flow into a mixing section provided on a downstream side, the ejector comprising: the flow path of the nozzle including a narrowing flow path in which the cross-sectional area of the flow path gradually decreases toward the downstream side, a constant-cross-section flow path having a substantially constant cross-sectional shape while extending from a downstream end of the narrowing flow path, the constant-cross-section flow path being continuous for a predetermined length, and a widening flow path continuous with a downstream end of the constant-cross-section flow path and in which the cross-sectional area of the flow path gradually increases toward the downstream side, the narrowing flow path takes in the motive fluid in a liquid state and allows the motive fluid in the liquid state to flow into the constant-cross-section flow path while reducing the pressure of the motive fluid in the liquid state, and the constant-cross-section flow path allows the motive fluid in the liquid state start to foam at a midway point of the predetermined length, characterized in that the length of the constant-cross-section flow path is selected from a length of the constant-cross-section flow path in which the flow rate of the constant-cross-section flow path equals the reference flow rate when the diameter of the throat portion is 1.1-fold to 1.2-fold of the reference diameter, the reference flow rate is an assumed flow rate through the narrowing flow path and the widening flow path that are joined directly without the constant-cross-section flow path in between, the reference diameter is a minimum diameter of the throat portion when the narrowing flow path and the widening flow path are joined directly. the flow path of the nozzle including a narrowing flow path in which the cross-sectional area of the flow path gradually decreases toward the downstream side, a constant-cross-section flow path having a substantially constant cross-sectional shape while extending from a downstream end of the narrowing flow path, the constant-cross-section flow path being continuous for a predetermined length, and a widening flow path continuous with a downstream end of the constant-cross-section flow path and in which the cross-sectional area of the flow path gradually increases toward the downstream side, a narrowing flow path in which the cross-sectional area of the flow path gradually decreases toward the downstream side, a constant-cross-section flow path having a substantially constant cross-sectional shape while extending from a downstream end of the narrowing flow path, the constant-cross-section flow path being continuous for a predetermined length, and a widening flow path continuous with a downstream end of the constant-cross-section flow path and in which the cross-sectional area of the flow path gradually increases toward the downstream side, the narrowing flow path takes in the motive fluid in a liquid state and allows the motive fluid in the liquid state to flow into the constant-cross-section flow path while reducing the pressure of the motive fluid in the liquid state, and the constant-cross-section flow path allows the motive fluid in the liquid state start to foam at a midway point of the predetermined length, characterized in that the length of the constant-cross-section flow path is selected from a length of the constant-cross-section flow path in which the flow rate of the constant-cross-section flow path equals the reference flow rate when the diameter of the throat portion is 1.1-fold to 1.2-fold of the reference diameter, the reference flow rate is an assumed flow rate through the narrowing flow path and the widening flow path that are joined directly without the constant-cross-section flow path in between, the reference diameter is a minimum diameter of the throat portion when the narrowing flow path and the widening flow path are joined directly.