A musical tone control system for an electronic keyboard instrument, which is capable of calculating a key depression velocity of a key with high accuracy without being adversely affected by a large rotational resistance of the key during a let-off, and stopping sounding of a musical tone in appropriate timing. The system detects first key depression information on a key at a first position corresponding to a key depression depth obtained before start of a let-off, second key depression information on the key at a second position which corresponds to a larger key depression depth than the first position does, and third key depression information, obtained after termination of the let-off, on the key at a third position which corresponds to a larger key depression depth than the second position does. The tone volume of a musical tone is set based on a key depression velocity of the key.