A printing unit, includes a head unit including: a thermal head; and a platen unit combined with the head unit. The platen unit including: a platen roller configured to feed a recording sheet while sandwiching the recording sheet together with the thermal head; a platen frame configured to support the platen roller in a rotatable manner; a sensor unit including: a sensor configured to detect the recording sheet; a connector connected to the sensor; and a connection portion configured to connect the sensor and the connector to each other; and a sensor holder, which is mounted to the platen frame, and includes: a plurality of connector arrangement portions through which a terminal of the connector is to be exposed; and a plurality of sensor receiving portions in which the sensor unit is to be received.