When coded image data which are obtained by coding image data by a coding method including an inter-frame predictive coding process are decoded, even when the decoding is started from an inter-frame predictive coded frame, the display of a decoded image having few disturbances is started, without the display being considerably delayed from the timing of input of coded image data. The encryption and the decryption may be performed using a bitwise XOR operation. In certain embodiments, the present invention allows target nucleic acid sequence detection in pooled biological samples without prior amplification of the target.