A sheet stacking device, comprising: a device main body; a sheet discharge unit arranged in the device main body and configured to discharge sheets; and a sheet stacking portion arranged in the device main body and including a sheet stacking surface on which the sheets discharged from the sheet discharge unit are to be stacked; an opening formed in the sheet stacking surface; and a sheet detector including a detection piece capable of projecting upwardly of the sheet stacking surface from and retracting into the opening, characterized by the sheet detector being capable of detecting a first state where no sheet is placed on the sheet stacking surface and the detection piece projects upwardly from the opening, a second state where a first number of the sheets discharged from the sheet discharge unit are placed on the sheet stacking surface and a third state where a second number of the sheets, the second number being larger than the first number, are placed on the sheet stacking surface, the detection piece is pressed downward by the weight of the sheets and the detection piece is retracted, wherein the sheet detector includes: a first sheet detector provided with a rotatable actuator including the detection piece and capable of detecting the first and second states according to the rotation of the actuator; and a second sheet detector rotatably supporting the first sheet detector and capable of detecting the third state according to the rotation of the first sheet detector. a device main body; a sheet discharge unit arranged in the device main body and configured to discharge sheets; and a sheet stacking portion arranged in the device main body and including a sheet stacking surface on which the sheets discharged from the sheet discharge unit are to be stacked; an opening formed in the sheet stacking surface; and a sheet detector including a detection piece capable of projecting upwardly of the sheet stacking surface from and retracting into the opening, characterized by the sheet detector being capable of detecting a first state where no sheet is placed on the sheet stacking surface and the detection piece projects upwardly from the opening, a second state where a first number of the sheets discharged from the sheet discharge unit are placed on the sheet stacking surface and a third state where a second number of the sheets, the second number being larger than the first number, are placed on the sheet stacking surface, the detection piece is pressed downward by the weight of the sheets and the detection piece is retracted, wherein the sheet detector includes: a first sheet detector provided with a rotatable actuator including the detection piece and capable of detecting the first and second states according to the rotation of the actuator; and a second sheet detector rotatably supporting the first sheet detector and capable of detecting the third state according to the rotation of the first sheet detector. a first sheet detector provided with a rotatable actuator including the detection piece and capable of detecting the first and second states according to the rotation of the actuator; and a second sheet detector rotatably supporting the first sheet detector and capable of detecting the third state according to the rotation of the first sheet detector.