To provide a liquid ejecting apparatus and a method for detecting a medium edge position in a liquid ejecting apparatus, whereby the position of an edge of a medium can be detected while also avoiding an event where a protrusion present in a region targeted for detection by an optical sensor is erroneously detected as being the medium, a carriage is moved so as to traverse a sheet of paper in the width direction, and a determination is made as to whether or not an output voltage of a sheet width sensor provided to the carriage has crossed over a first threshold value.