1. A digital content playback apparatus which generates new digital content by replacing one or more content parts included in digital content by one or more other content parts, the digital content playback apparatus comprising: content storage configured to store a plurality of content parts, license information storage configured to store license information describing license conditions defined for the content parts, each of which includes at least a replacement permission condition and a replacement target specification condition among conditions in which the replacement permission condition defines that a content part which has replacement permission is replaceable by another content part, the replacement target specification condition specifies content parts which are replacement target candidates when the replacement permission condition is replaceable, a complementary relationship condition specifies another content part that must be contained in digital content when using the content part which has replacement permission, and an automatic replacement permission condition specifies whether to allow replacement of the content part which has automatic replacement permission without user instruction, a content part selecting unit configured to allow a user to specify a replacement source content part as a content part that should be replaced among content parts in the digital content including content parts in the content storage, and a replacement target content part as a content part for adding to the digital content for replacing the replacement source content part, verifying unit configured to verify whether license conditions of content parts in the digital content and the replacement target content part would be satisfied when replacing the replacement source content part by the replacement target content part, and automatically replace the content part to satisfy complementary relationship conditions of all content parts, a content generation unit configured to generate new digital content by replacing the replacement source content part by the replacement target content part when the license conditions are determined as being satisfied by the verifying unit, and a content playback unit configured to play back the generated new digital content, wherein the content part selecting unit presents to the user content parts whose replacement permission condition is replaceable and whose automatic replacement permission condition is impermissible as candidates for the replacement source content part among content parts in the digital content and content parts specified by replacement target specification conditions of the candidates for the replacement source content part as candidates for the replacement target content part.