To easily mount and remove a switch without disassembling it.

The switch state detection device comprises a first stationary member detachably fixed to a support part supporting a main shaft of the switch, a resistive element arranged on an outer surface of the first stationary member, a second stationary member having an arm extending on the resistive element and detachably fixed to the main shaft, a contact piece slidably mounted to the arm of the second stationary member into contact with the resistive element, and a detection part electrically connected to the resistive element and the contact piece, measuring variation of a resistance value of the resistive element corresponding to driving of the main shaft, and detecting an operational condition of a contact.