A powder supply method for supplying a powder to a sheet being conveyed along a conveyance path, the powder supply method implementing: a distribution step of dropping the powder from a storage section storing therein the powder, to distribute the powder onto a surface of the sheet through a distribution port; a first closing step of moving a first opening-closing device such that the first opening-closing device periodically overlaps the distribution port when viewed from vertically above the distribution port, to periodically close the distribution port by the first opening-closing device; a second closing step of moving a second opening-closing device such that the second opening-closing device overlaps the distribution port when viewed from vertically above the distribution port, in a same cycle period as that of the first opening-closing device and at a timing later than that of the first opening-closing device, to periodically close the distribution port by the second opening-closing device, and moving the second opening-closing device such that the second opening-closing device extends from an upstream-side edge of the first opening-closing device lying on an upstream side of a movement direction of the first opening-closing device toward the upstream side of the movement direction of the first opening-closing device, during a time period during which the upstream-side edge of the first opening-closing device overlaps the distribution port, when viewed from vertically above the distribution port; and a timing setting step of setting a difference between a timing at which the second opening-closing device overlaps the distribution port when viewed from vertically above the distribution port, and a timing at which the first opening-closing device overlaps the distribution port when viewed from vertically above the distribution port. a distribution step of dropping the powder from a storage section storing therein the powder, to distribute the powder onto a surface of the sheet through a distribution port; a first closing step of moving a first opening-closing device such that the first opening-closing device periodically overlaps the distribution port when viewed from vertically above the distribution port, to periodically close the distribution port by the first opening-closing device; a second closing step of moving a second opening-closing device such that the second opening-closing device overlaps the distribution port when viewed from vertically above the distribution port, in a same cycle period as that of the first opening-closing device and at a timing later than that of the first opening-closing device, to periodically close the distribution port by the second opening-closing device, and moving the second opening-closing device such that the second opening-closing device extends from an upstream-side edge of the first opening-closing device lying on an upstream side of a movement direction of the first opening-closing device toward the upstream side of the movement direction of the first opening-closing device, during a time period during which the upstream-side edge of the first opening-closing device overlaps the distribution port, when viewed from vertically above the distribution port; and a timing setting step of setting a difference between a timing at which the second opening-closing device overlaps the distribution port when viewed from vertically above the distribution port, and a timing at which the first opening-closing device overlaps the distribution port when viewed from vertically above the distribution port.