A performance data editing system is actualized by a computer system (or electronic musical instrument) which is equipped with a display and a mouse. The system initially provides a score window containing various types of execution icon layers onto which execution icons (representing musical symbols such as bend-up/down, grace-up/down, dynamics, glissando, tremolo) are attached and arranged in conformity with a progression of a musical tune on a screen of the display. Each of the layers is independently controlled in response to various commands such as display-on, small-scale display, display-off and vertical rearrangement. The system 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 system also allows the user to modify parameters of a specific icon which is selected from among the execution icons attached onto the score window. That is, the user opens an icon modify window to change parameters of the specific icon with the mouse. Further, the system provides the user with a simple operation for deletion of execution-related data from performance data. That is, when the user performs drag-and-drop operations on a certain execution icon to move it outside of a prescribed display area (e.g., layer window) of the score window, the system automatically deletes the corresponding execution-related data from the performance data. Thus, it is possible to improve performability and efficiency in editing performance data by using icons with simple operations and without errors.