A musical instrument includes memory that stores layout information defining regions arranged on a predetermined virtual plane, and a position sensor that detects the position coordinates on the virtual plane of a music playing member that can be held by a player. First, it is determined whether the position coordinates of the music playing member belong to a region arranged on the virtual plane based on the layout information, at a timing at which a specific music playing operations is made. Herein, in a case of having determined as belonging to a region, the generation of sound of a musical note corresponding to this region is instructed, in a case of having determined as not belonging to a region, the layout information stored in the memory is modified in order to modify this region so as to include the position coordinates of the music playing member.