The yield of a product is improved when a substrate held by a conveyance carrier is subjected to a plasma treatment. A plasma treatment method of the substrate held by the conveyance carrier includes preparing the conveyance carrier which includes a holding sheet and a frame disposed on the outer peripheral portion of the holding sheet; bonding the substrate to the holding sheet so that the substrate is held by the conveyance carrier; and increasing tensile strength of the holding sheet. The plasma treatment method further includes placing the conveyance carrier on the stage after the bonding of the substrate and bringing the substrate into contact with the stage through the holding sheet; and performing a plasma treatment on the substrate after the placing of the conveyance carrier.