In an electronic keyboard instrument, each key moves from an initial position to a full stroke position via first, second, and third positions sequentially. A velocity detector detects a first key velocity of the key when the key moves from the first position to the second position, and detects a second key velocity of the key when the key further moves from the second position to the third position. A predictor obtains a predicted key-on time and a predicted key velocity based on the first key velocity, provided that the first key velocity is greater than a predetermined threshold velocity. A tone generator generates a musical sound signal based on the first key velocity and the predicted key velocity if the second key velocity is not detected before the predicted key-on time.