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, comprising: a content storage configured to store a plurality of content parts, a 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 the replacement permission condition defining whether the content part is replaceable by an other content part, the replacement target specification condition specifying content parts being candidates of replacement target when the replacement permission condition is replaceable and a complementary relationship condition specifying an other content part that must be contained in digital content when using the content part, 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 some 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, a 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, a content generation unit configured to generate new digital content by replacing the replacement source content part by the replacement target content part when determined as being satisfied by the verifying unit; and a content playback unit configured to play back the generated new digital content.