A musical keyboard includes a plurality of white keys and black keys. As a number of keys are depressed simultaneously, a pattern of the depressed keys is judged as categorized in a number of predetermined patterns of depressed keys in view of the white keys and the black keys based on the input note data acquired from the depressed keys. A plurality of types of harmonization are predetermined, from among which a type of harmonization is determined. According to the determined type of harmonization, the input note data representing the notes of the depressed keys are adjusted so that inadequate notes included in the input notes are changed to adequate notes to constitute a chord as permitted by the type of harmonization in view of musical grammar. An unskilled player can play music incorporating improvisation which would need higher musical skill, as the inadvertently erroneous depression of the keys is rectified in the note data processing.