A battery-operated object detecting device for detecting an object and outputting a detection information signal, which object detecting device comprises: a detecting unit (1A, 1B, 2A, 2B) adapted to be driven by an electric cell mounted thereon; and a transmitting device without its own electric cell and driven by the electric cell in the detecting unit (1A, 1B, 2A, 2B) for transmitting wirelessly at least a low battery signal from the detecting unit (1A, 1B, 2A, 2B) and a detection information signal from the detecting unit, the low battery signal being indicative of a reduction in a supply voltage of the electric cell, wherein the detecting unit (1A, 1 B, 2A, 2B) comprises a first low battery detecting circuit built therein for detecting a low battery state, in which the voltage of the electric cell in the detecting unit (1A, 1B, 2A, 2B) is lower than a first reference value, wherein the transmitting device comprises a second low battery determining circuit built therein for determining whether or not the voltage supplied from the electric cell is lower than a second reference value which is lower than the first reference value, wherein the object detecting device further comprises an electric power sharing unit for outputting a supply voltage towards the transmitting device in response to the voltage from the electric cell, and wherein the electric power sharing unit comprises a voltage changing circuit for changing the supply voltage from the electric cell based on a low battery signal from the first low battery detecting circuit indicative of a voltage lower than the first reference value, to a voltage necessary for causing the second low battery determining circuit of the transmitting device to determine a low battery state that is lower than the second reference value; so as to cause the transmitting device to transmit the low battery signal indicative of a voltage lower than the second reference value to indicate the low battery state of the detecting unit; and wherein the voltage changing circuit is provided with a plurality of change voltages, and in which the electric power sharing unit comprises a changeover switch for changing the change voltage by the voltage changing circuit in dependence on a type of the transmitting device used. a detecting unit (1A, 1B, 2A, 2B) adapted to be driven by an electric cell mounted thereon; and a transmitting device without its own electric cell and driven by the electric cell in the detecting unit (1A, 1B, 2A, 2B) for transmitting wirelessly at least a low battery signal from the detecting unit (1A, 1B, 2A, 2B) and a detection information signal from the detecting unit, the low battery signal being indicative of a reduction in a supply voltage of the electric cell, wherein the detecting unit (1A, 1 B, 2A, 2B) comprises a first low battery detecting circuit built therein for detecting a low battery state, in which the voltage of the electric cell in the detecting unit (1A, 1B, 2A, 2B) is lower than a first reference value, wherein the transmitting device comprises a second low battery determining circuit built therein for determining whether or not the voltage supplied from the electric cell is lower than a second reference value which is lower than the first reference value, wherein the object detecting device further comprises an electric power sharing unit for outputting a supply voltage towards the transmitting device in response to the voltage from the electric cell, and wherein the electric power sharing unit comprises a voltage changing circuit for changing the supply voltage from the electric cell based on a low battery signal from the first low battery detecting circuit indicative of a voltage lower than the first reference value, to a voltage necessary for causing the second low battery determining circuit of the transmitting device to determine a low battery state that is lower than the second reference value; so as to cause the transmitting device to transmit the low battery signal indicative of a voltage lower than the second reference value to indicate the low battery state of the detecting unit; and wherein the voltage changing circuit is provided with a plurality of change voltages, and in which the electric power sharing unit comprises a changeover switch for changing the change voltage by the voltage changing circuit in dependence on a type of the transmitting device used.