The present inventors identified the causative gene of pyl mutant, which is mutable and shows a lowered chlorophyll accumulation, and discovered that a new type of transposon classified to Ac/Ds type is involved in the mutation. It is confirmed for the first time that the Ac/Ds type transposon nDart (SEQ ID NO: 1) in rice has transposon activity under normal cultivation conditions. Furthermore, an autonomous element Dart was discovered by analyzing said Ac/Ds type transposon.