A batteryless absolute encoder determines the accuracy of absolute position information to be output when electric power is turned on and outputs an alarm signal in the case of abnormality. The batteryless absolute encoder includes an absolute position computing section, an absolute position storing section, and a determining section. The absolute position computing section computes the absolute position of a spindle to be detected, including the number of revolutions of the spindle, based upon detection signals output from four reluctance resolvers. The absolute position storing section stores the absolute position output from the absolute position computing section when electric power is turned off. The determining section compares an absolute position output from the absolute position computing section when electric power is turned on and the stored absolute position and outputs an alarm signal if a difference between the two absolute positions is larger than a predetermined value.