In an electronic musical instrument, the keys of a keyboard are divided into a plurality of groups by octaves, and a wave generator is provided with a memory for storing constants corresponding to musical tone frequencies to be generated according to the notes includes in a desired group out of the plurality of group, an accumulator for repeatedly adding the constants read out of the memory, and a bit position shifting circuit for shifting the bit position of the data produced by the accumulator according to the octave range of a note to be produced, thereby to obtain the data varying repeatedly according to the frequency of a note to be produced, and to reduce the storing capacity of the memory when compared to the number of keys.