Abstract: A biological sensing device for sensing and estimating the presence of quantity of living things in a sensing space is disclosed to include a controller, a microprocessor, a power source and a sensing unit including an electrode set for creating electrostatic coupling in the sensing space and a sensor controlled by the controller to sense a capacitance change in the sensing space indicative of the presence and quantity of living things in the sensing space and to transmit a corresponding signal to the microprocessor for computing so that the microprocessor can estimate the presence and quantity of living things in the sensing space.