An output changing method of an A/D conversion apparatus is provided. The apparatus includes a pulse delay circuit in which delay units are connected in series, and an encoding circuit which detects the number of stages of the delay units, through which a pulse signal passes during predetermined measurement time, and generates numeric data corresponding to the number of stages. The apparatus receives an analog input signal as power supply voltage of the pulse delay circuit to perform A/D conversion for the analog input signal. The method includes determining whether or not the analog input signal is within an allowable voltage range in which the apparatus operates normally, outputting the numeric data as an A/D conversion value if the analog input signal is within the range, and outputting numeric data formed of a specified value as the A/D conversion value if the analog input signal is not within the range.