1. A screen printer comprising: a main body comprising an operation region in which a stencil mask is installable, and at least one introduction and extraction portion through which the stencil mask is introduced into the operating region from outside of the main body and is extracted from the operation region to outside of the main body; and a conveyance unit which conveys the stencil mask introduced into and installed at the operation region to the at least one introduction and extraction portion when the stencil mask is replaced by a new stencil mask introduced through the at least one introduction and extraction portion, and conveys the new stencil mask to the operation region.