The invention discloses a screen printer, a component mounting line, and a screen printing method. The screen printer includes: a substrate holder that holds a substrate; a mask to be contacted with the substrate; an imaging device that images the substrate; an elevation mechanism that moves up the substrate holder at a below-mask-position below the mask based on a result of the imaging to contact the substrate with the mask; a horizontal moving mechanism that moves the substrate holder between the below-mask-position and a retreat position retreated in a horizontal direction from the below-mask-position; a printing head that moves on the mask contacted with the substrate and prints a paste on the substrate via the mask; and a mask cleaner that cleans a lower surface of the mask with the substrate separated from the mask by the elevation mechanism. The imaging of the substrate and the cleaning of the mask are performed with the substrate holder positioned at the retreat position.