To suppress size increase of a structure while providing a mechanism for preventing falling-off of a button battery in a medical device. A detection unit detects biological information of a subject. A housing defines an accommodating chamber for accommodating a button battery for supplying electric power to the detection unit. A cover is capable of closing the accommodating chamber. A locking member is supported by the housing and locks the cover to the housing so as to close the accommodating chamber. A space allowing displacement of the locking member for relating locking is formed by the housing and the cover.
