A battery-powered, electronic instrument designed to continuously measure (monitor) a patient's body temperature (e.g., during the administration of general anaesthesia, treatment of hypothermia and hyperthermia). It comprises an electronic unit and an attached probe that detects and converts the changes in temperature into variations of an electric characteristic, e.g., resistance or voltage. These variations of the electric characteristic are processed in the electronic circuits and continuously displayed as temperature readings. This is a reusable device.