A short PN code generating device in a mobile communication network having a N× CDMA communication system performing spreading at an N× chip rate using a short PN code of chip length N×2k and a 1× CDMA communication system performing spreading at a 1× chip rate using a short PN code of chip length 2k. The device generates a short PN code such that the N× CDMA communication system can distinguish at least the same number of base stations as the 1× CDMA communication system can distinguish. The device includes a sequence generator for generating an m-sequence having a chip length longer than the chip length N×2k to output an I-component short PN code; and a short PN code generator for switching the generated m-sequence at predetermined chip periods to alternately output the m-sequence as an I-component short PN code and a Q-component short PN code.