A multivalued FSK modulation system is provided in which when data to be transmitted is multivalued and transmitted/received, the detection level is different every symbol. When a two-valued signal is transmitted as one symbol, data (11), (01), (00) and (10) are previously set to be, for example, a shift of -6, a shift of +6, a shift of +2, and a shift of -2, respectively. When data to be input next is (00), mapping is performed so as to provide a sign weight of +2 from the sign weight at the position of current data. When the data to be input next is (10), mapping is performed so as to provide a sign weight of -2 from the position of the current sign weight. Thus, even if data of the same level is input, signals can be always detected in different levels, so that the sign detection point is not specified erroneously.