In an electronic musical apparatus having an acoustic instrument manually operable to commence an acoustic vibration and a tone generator responsive to the acoustic vibration to generate a musical tone having a pitch corresponding to that of the acoustic vibration, a pitch detecting device utilizes a pickup (3) for picking up the acoustic vibration to convert the same into a waveform signal. Further, a first detector (13) operates according to a fast algorithm for processing the waveform signal so as to responsively produce a first output representative of the pitch of the acoustic vibration, and a second detector (12) operates in parallel to the first detector (13) for processing the same waveform signal according to a slow algorithm so as to stably produce a second output representative of the pitch of the acoustic vibration. A selector (17) selectively feeds one of the first output and the second output to the tone generator so that the first detector (13) and the second detector (12) can cooperate complementarily with each other to ensure responsive and stable detection of the pitch of the acoustic vibration. An additional detector (15) processes the waveform signal to measure a time interval between a pair of the peaks so as to detect a plucking point. A controller (21) controls the tone generator according to the detected plucking point to change the timbre of the tone generator in response to the plucking point.