To provide encoder count error detection circuitry and an encoder count error detection method capable of realizing a highly reliable encoder by providing detection of the count errors of count signals used to measure displacement, an encoder count error detection circuitry and detection method configured as count error detection circuitry for an encoder outputting pulse trains according to the displacement of a measured object, doubles, triples, or quadruples one or more fundamental signals used for measuring displacement to convert to count signals, and compares the output signals of the counter counting these count signals with the level of these fundamental signals to detect count errors in the count signals.