Microcomputers mounted on communication modules in IoT devices have low impedance and poor efficiency when the power supply voltage is low. In a weak power supply by energy harvesting, power consumption increases due to deterioration of efficiency at low voltage. Therefore, the DC voltage required to operate the ZigBee module, etc., cannot be obtained, causing a severe problem that startup cannot be performed. To solve the problem, we realized a startup management circuit to cut off the power supply to the ZigBee module until the DC voltage reaches the microcomputer's operating voltage. The proposed system starts at around 2.3 V by using the rectenna operating at 920 MHz band RF signal.