Data for each sample point amplitude values of a desired waveshape is expressed in mantissa and exponent in accordance with a floating-point representation. The desired waveshape is divided into a plurality of frames along a time axis and the exponent data is common in each individual one of said frames which includes a plurality of sample points. The number of the frames is much smaller than the number of all sample points. In this waveshape memory, the mantissa data corresponding to the respective sample points are stored in a first memory, and exponent data corresponding to the respective frames are stored in a second memory.