To solve the problem that e.g. EFM modulated signals of less than 3T are detected and decoded into data of normal lengths but the system clock frequency is high to result in a high power consumption with much heating.

The decoder decodes codes having a minimum inversion interval 3T where T is the bit spacing and the continuous length nT of other symbols successively inserted between identical symbols in a code series composed of two symbols is defined as a specified length. The decoder has a means 31 for converting specified serial data of a code series into a plurality M of parallel data, a means 64 for detecting a pattern of T normally inexistent in normal data with inversion intervals of 3T or longer from the code series, and means 62A, 62B for correcting the pattern into signals of a normal format with the parallel data when the detecting means 64 detects the normally inexistent pattern.