an observed image forming apparatus comprises : an apparatus body including a plurality of connector receivers to which electric connectors of a plurality of observation devices for obtaining an image of an object to be observed can be connected ; a cap that is to be mounted to one of said plurality of connector receivers to which any of said plurality of electric connectors is not connected ; and a cap insertion and extraction detecting circuit that detects whether or not the cap is mounted to the one of the connector receivers in the apparatus body , wherein , when mounting of the cap has been detected by the cap insertion and extraction detecting circuit , one of the observation devices an electric connector of which is connected to the other one of the connector receivers in the apparatus body is controlled to be driven .