A fixing portion for an image forming apparatus comprising: a heater for heating a toner image transferred onto a paper sheet; a first power line for connecting the heater to an AC power supply so that power is supplied to one end of the heater; a second power line for connecting the heater to the AC power supply so that power is supplied to the other end of the heater; a switch portion disposed in the first power line between the heater and the AC power supply so as to turn on and off power supply; an overheat protection portion disposed in the second power line between the heater and the AC power supply so as to disconnect the second power line when temperature exceeds a predetermined temperature; a zero cross signal generation portion which outputs a zero cross signal on the basis of a voltage of the first power line between the switch portion and the heater and a voltage of the second power line between the overheat protection portion and the heater in accordance with a waveform of the AC power supply, a recognition portion which is supplied with an output value of the zero cross signal generation portion so as to recognize to be one of states in accordance with the output value, characterized in that the zero cross signal generation portion is configured to output zero cross signals having different output values in a case where the voltage of the first power line is higher than the voltage of the second power line and in a case where the voltage of the second power line is higher than the voltage of the first power line, the zero cross signal generation portion includes a light emitting portion for emitting light on the basis of current supplied through a first resistor connected between the switch portion and the heater and current supplied through a second resistor connected between the overheat protection portion and the heater, and a light receiving portion for receiving light emitted from the light emitting portion so as to output different values in accordance with light intensity of the light emitting portion, the first resistor and the second resistor have different resistance values, and the states include a normal power supply state in which power from the AC power supply is normally supplied, a state where the power is turned off by the switch portion, a state where the second power line is disconnected by the overheat protection portion, and a power supply stop state. a heater for heating a toner image transferred onto a paper sheet; a first power line for connecting the heater to an AC power supply so that power is supplied to one end of the heater; a second power line for connecting the heater to the AC power supply so that power is supplied to the other end of the heater; a switch portion disposed in the first power line between the heater and the AC power supply so as to turn on and off power supply; an overheat protection portion disposed in the second power line between the heater and the AC power supply so as to disconnect the second power line when temperature exceeds a predetermined temperature; a zero cross signal generation portion which outputs a zero cross signal on the basis of a voltage of the first power line between the switch portion and the heater and a voltage of the second power line between the overheat protection portion and the heater in accordance with a waveform of the AC power supply, a recognition portion which is supplied with an output value of the zero cross signal generation portion so as to recognize to be one of states in accordance with the output value, characterized in that the zero cross signal generation portion is configured to output zero cross signals having different output values in a case where the voltage of the first power line is higher than the voltage of the second power line and in a case where the voltage of the second power line is higher than the voltage of the first power line, the zero cross signal generation portion includes a light emitting portion for emitting light on the basis of current supplied through a first resistor connected between the switch portion and the heater and current supplied through a second resistor connected between the overheat protection portion and the heater, and a light receiving portion for receiving light emitted from the light emitting portion so as to output different values in accordance with light intensity of the light emitting portion, the first resistor and the second resistor have different resistance values, and the states include a normal power supply state in which power from the AC power supply is normally supplied, a state where the power is turned off by the switch portion, a state where the second power line is disconnected by the overheat protection portion, and a power supply stop state.