An operation apparatus (1A, 1B) including an input device and a display device (3A, 3B) for operating a function of a control target apparatus, characterized in that the display device (3A, 3B) comprises: a detection unit (3b-1) for detecting whether the input device and the display device (3A, 3B) are in a combined state or in a detached state; a determining unit for determining a control mode associated with a state detected by the detection unit from among control modes that can be set for the control target apparatus, the state being one of the combined state and the detached state; and a control processing unit (3f-1, 3f-2) for switching a control mode of the control target apparatus to the control mode determined by the determining unit and for causing the display device to display a function to be operated in a display mode corresponding to the switched control mode wherein a simple control mode is associated with the detached state and a full control mode is associated with the combined state, wherein the detection unit is configured to detect a positional relationship between the input device and the display device in the combined state, and, in the full control mode, the control processing unit is configured to cause the display device to display the function to be operated in the control mode determined by the determining unit in a display mode associated with the positional relationship detected by the detection unit, and accepts an operation of the function by an operation method associated with the display mode, wherein, in the simple control mode, the control processing unit is configured to select only a preset function among functions that can be operated in the control target apparatus and cause the display device to display the selected function. a detection unit (3b-1) for detecting whether the input device and the display device (3A, 3B) are in a combined state or in a detached state; a determining unit for determining a control mode associated with a state detected by the detection unit from among control modes that can be set for the control target apparatus, the state being one of the combined state and the detached state; and a control processing unit (3f-1, 3f-2) for switching a control mode of the control target apparatus to the control mode determined by the determining unit and for causing the display device to display a function to be operated in a display mode corresponding to the switched control mode wherein a simple control mode is associated with the detached state and a full control mode is associated with the combined state, wherein the detection unit is configured to detect a positional relationship between the input device and the display device in the combined state, and, in the full control mode, the control processing unit is configured to cause the display device to display the function to be operated in the control mode determined by the determining unit in a display mode associated with the positional relationship detected by the detection unit, and accepts an operation of the function by an operation method associated with the display mode, wherein, in the simple control mode, the control processing unit is configured to select only a preset function among functions that can be operated in the control target apparatus and cause the display device to display the selected function.