In a first sensing state in which a first current flows in a forward direction with respect to a pn junction of a first semiconductor element and a second current of a different magnitude from the first current flows in a forward direction with respect to a pn junction of a second semiconductor element, a difference between a forward direction voltage of the pn junction of the first semiconductor element and a forward direction voltage of the pn junction of the second semiconductor element is converted into a digital value by a computer and acquired as a first digital value. In a second sensing state in which the second current flows in the forward direction in the pn junction of the first semiconductor element and the first current flows in the forward direction in the pn junction of the second semiconductor element, a difference between the forward direction voltage of the pn junction of the first semiconductor element and the forward direction voltage of the pn junction of the second semiconductor element is converted into a digital value by the computer and acquired as a second digital value. A temperature measurement value is computed based on an average value of the first digital value and the second digital value by the computer.