A platen gap adjustment mechanism includes a carriage guide shaft, a carriage drive unit and a main carriage. The carriage guide shaft is configured to extend perpendicularly to a media conveyance direction that passes over a platen. The carriage drive unit is supported on the carriage guide shaft and pivotable around an axis of the carriage guide shaft. The main carriage is supported on the carriage drive unit, pivotable around a parallel axis that is parallel to the axis of the carriage guide shaft, and configured to carry a printhead for printing on a print medium conveyed along the media conveyance path past the platen. The main carriage is moveable toward and away from the platen according to a thickness of the print medium while being slidable in sliding contact with the print medium conveyed over the platen to maintain a constant gap between the printhead and the print medium.