A roll substrate holding device has improved versatility of a winding core and sufficient holing strength. The roll substrate holding device that holds a roll substrate wound around a winding core without a shaft includes: a holding member that is inserted into an opening of a first winding core having a first inner diameter and that comes into contact with the first winding core to rotatably hold the first winding core; and a conversion adapter that is mounted in an opening of a second winding core having a second inner diameter larger than the first inner diameter, wherein the holding member is inserted into an opening of the conversion adapter that is mounted in advance on the second winding core, and comes into surface contact or line contact with an inner peripheral surface of the conversion adapter having the first inner diameter to rotatably hold the second winding core.