An encoder according to an embodiment of this disclosure includes a voltage generation circuit connected to a power supply through a diode and having a variable resistor, the voltage generation circuit outputting a voltage corresponding to a current flowing through the diode and a resistance value of the variable resistor, as a threshold value; a comparator for comparing an analog signal inputted from a detector for detecting rotation of a motor with the threshold value inputted from the voltage generation circuit, and outputting a comparison result as a comparator output; an A/D converter for converting the analog signal into a digital signal; a threshold value determination circuit for calculating a new threshold value using the digital signal; and a resistance value change circuit for changing a resistance value of the variable resistor, such that the calculated new threshold value is inputted from the voltage generation circuit to the comparator.