An electronic musical instrument is of a type wherein musical tone waveforms are stored in a memory as their sampled amplitudes and sequentially and repetitively read out to constitute tone waveforms. A key depression brings forth frequency information in a digital representation. The frequency information is accumulated to make an address signal for reading out the waveform memory. When a depressed key is moved laterally, a touch detection circuit produces an analog detection signal representing the amount of displacement of the key. In the meanwhile, a clock pulse is counted by an up-down counter and the counting output is converted by a D-A converter to an analog signal to obtain a triangular wave function signal. When the analog detection signal coincides, in amplitude, with the triangular wave function signal, the digital signal from the up-down counter is sampled and held by a sample and hold circuit. Thus a digital signal representing the key displacement is obtained. This digital signal is multiplied with a frequency information in a multiplier and, accordingly, the musical tone frequency is modulated in response to the lateral movement of the key. Thus, the touch vibrato effect is obtained.