A device for measuring human body impedance, comprising a pair of input electrodes and a pair of output electrodes, a constant-current source, a voltage measurement unit and an operation processing unit The voltage measurement unit measures the voltage generate by the current from the constant-current source passing through the human body, and the operation processing unit calculates the human body impedance according to correction parameters. When in use, the comparison with the measurement of the standard resistor and the switching of electronic analog switches is omitted, which greatly reduces the period of measurement lowers the cost of the product simplifies the circuit strengthens the anti-interference capability of the circuit, and cuts down the power consumption of the circuit A human body fat meter using the device for measuring the human body impedance is also disclosed, which quickly and precisely converts the impedance into fat content of the human body.