A printing unit, comprising: a head unit including a thermal head configured to perform printing on a recording sheet; a platen unit including: a platen roller which extends in a first direction, and is configured to feed the recording sheet; and a pair of platen bearings configured to support both end portions of the platen roller in the first direction in a rotatable manner, the platen unit being detachably combined with the head unit by allowing the pair of platen bearings to be inserted in and disengaged from a pair of receiving grooves formed in the head unit and recessed toward one side in a second direction orthogonal to the first direction; an operation lever which is swingable about a first swing shaft and a first rotational axis extending in the first direction between a lock position of locking the platen unit to the head unit and an unlock position of unlocking the platen unit from the head unit; a lock arm which is swingable about a second swing shaft and a second rotational axis extending in the first direction between a lock state of locking the platen roller under a state in which the pair of platen bearings are fitted in the pair of receiving grooves when the operation lever is at the lock position, and an unlock state of allowing the pair of platen bearings to be disengaged from the pair of receiving grooves through openings of the pair of receiving grooves toward another side in the second direction when the operation lever is at the unlock position; the lock arm configured to swing about the second rotational axis to the unlock state along with movement of the operation lever from the lock position to the unlock position, and a disengagement arm which is configured to be pivoted, along with pivoting of the operation lever from the lock position toward the unlock position, about a third swing shaft extending in the first direction at a position different from the second swing shaft and the first swing shaft, and is configured to press at least one of the pair of platen bearings from an inside of the receiving groove toward the opening side so as to disengage the at least one of the pair of platen bearings from the receiving groove. a head unit including a thermal head configured to perform printing on a recording sheet; a platen unit including: a platen roller which extends in a first direction, and is configured to feed the recording sheet; and a pair of platen bearings configured to support both end portions of the platen roller in the first direction in a rotatable manner, the platen unit being detachably combined with the head unit by allowing the pair of platen bearings to be inserted in and disengaged from a pair of receiving grooves formed in the head unit and recessed toward one side in a second direction orthogonal to the first direction; a platen roller which extends in a first direction, and is configured to feed the recording sheet; and a pair of platen bearings configured to support both end portions of the platen roller in the first direction in a rotatable manner, the platen unit being detachably combined with the head unit by allowing the pair of platen bearings to be inserted in and disengaged from a pair of receiving grooves formed in the head unit and recessed toward one side in a second direction orthogonal to the first direction; an operation lever which is swingable about a first swing shaft and a first rotational axis extending in the first direction between a lock position of locking the platen unit to the head unit and an unlock position of unlocking the platen unit from the head unit; a lock arm which is swingable about a second swing shaft and a second rotational axis extending in the first direction between a lock state of locking the platen roller under a state in which the pair of platen bearings are fitted in the pair of receiving grooves when the operation lever is at the lock position, and an unlock state of allowing the pair of platen bearings to be disengaged from the pair of receiving grooves through openings of the pair of receiving grooves toward another side in the second direction when the operation lever is at the unlock position; the lock arm configured to swing about the second rotational axis to the unlock state along with movement of the operation lever from the lock position to the unlock position, and a disengagement arm which is configured to be pivoted, along with pivoting of the operation lever from the lock position toward the unlock position, about a third swing shaft extending in the first direction at a position different from the second swing shaft and the first swing shaft, and is configured to press at least one of the pair of platen bearings from an inside of the receiving groove toward the opening side so as to disengage the at least one of the pair of platen bearings from the receiving groove.