A morphed musical piece generation system that generates a morphed musical piece between a first musical piece and a second musical piece, comprising: a common time-span tree data generation section that generates, on the basis of first time-span tree data on a first time-span tree obtained by analyzing first musical piece data on the first musical piece and second time-span tree data on a second time-span tree obtained by analyzing second musical piece data on the second musical piece, common time-span tree data on a common time-span tree obtained by extracting common information between the first time-span tree and the second time-span tree; a first intermediate time-span tree data generation section that generates, on the basis of the first time-span tree data and the common time-span tree data, first intermediate time-span tree data on a first intermediate time-span tree generated by selectively removing one or more pieces of difference information between the first time-span tree and the common time-span tree from the first time-span tree or selectively adding the one or more pieces of difference information to the common time-span tree; a second intermediate time-span tree data generation section that generates, on the basis of the second time-span tree data and the common time-span tree data, second intermediate time-span tree data on a second intermediate time-span tree generated by selectively removing one or more pieces of difference information between the second time-span tree and the common time-span tree from the second time-span tree or selectively adding the one or more pieces of difference information to the common time-span tree; a data combining section that generates, on the basis of the first intermediate time-span tree data and the second intermediate time-span tree data, combined time-span tree data on a combined time-span tree obtained by combining the first intermediate time-span tree and the second intermediate time-span tree; and a musical piece data generation section that generates, on the basis of the combined time-span tree data, musical piece data corresponding to the combined time-span tree as musical piece data on the morphed musical piece. a common time-span tree data generation section that generates, on the basis of first time-span tree data on a first time-span tree obtained by analyzing first musical piece data on the first musical piece and second time-span tree data on a second time-span tree obtained by analyzing second musical piece data on the second musical piece, common time-span tree data on a common time-span tree obtained by extracting common information between the first time-span tree and the second time-span tree; a first intermediate time-span tree data generation section that generates, on the basis of the first time-span tree data and the common time-span tree data, first intermediate time-span tree data on a first intermediate time-span tree generated by selectively removing one or more pieces of difference information between the first time-span tree and the common time-span tree from the first time-span tree or selectively adding the one or more pieces of difference information to the common time-span tree; a second intermediate time-span tree data generation section that generates, on the basis of the second time-span tree data and the common time-span tree data, second intermediate time-span tree data on a second intermediate time-span tree generated by selectively removing one or more pieces of difference information between the second time-span tree and the common time-span tree from the second time-span tree or selectively adding the one or more pieces of difference information to the common time-span tree; a data combining section that generates, on the basis of the first intermediate time-span tree data and the second intermediate time-span tree data, combined time-span tree data on a combined time-span tree obtained by combining the first intermediate time-span tree and the second intermediate time-span tree; and a musical piece data generation section that generates, on the basis of the combined time-span tree data, musical piece data corresponding to the combined time-span tree as musical piece data on the morphed musical piece.