The present invention is directed to a performance data editing system embodied within a computer system (or electronic musical instrument) equipped with a display and a mouse. The present invention provides a score window containing various types of execution icon layers onto which execution ions representing exection-related data for adding articulation to a music tone are attached and arranged in conformity with a progression of a musical tune on a screen of the display. Each layer can be controlled in response to various commands. The present invention further allows a user (or music editor) to select desired execution icons from an icon select palette that provides lists of execution icons which are registered in advance. In addition, the present invention allows a user to modify parameters of a specific icon, as well as add icons to or delete icons from a prescribed display area (e.g., layer window) using drag-and-drop operations.