To provide a malfunction detection device for detecting a malfunction of a joystick.

In the malfunction detection device for an operating means provided with a first and a second potentiometer 2, 3 which output voltages changing according to the amount of operation of the joystick 1 as an operating means respectively, and a controller 10 to which the outputs of the first and second potentiometers 2, 3 are input, reference voltages are supplied to the first and second potentiometers 2, 3 respectively. The first potentiometer 2 outputs a voltage in proportion to the amount of operation of the joystick 1, and the second potentiometer 3 outputs a voltage in inverse proportion to the amount of operation of the joystick 1. The controller 10 calculates the sum of the respective outputs of the first and second potentiometers 2, 3, and detects the malfunction of the joystick 1 on the basis of the calculated sum of the output voltages.