A thermal printer includes a body detachably including a platen roller unit having a platen roller, and a cover element movable relative to the body between an open position and a closed position, detachably including a thermal printhead unit. The cover element further includes a claw protruding backward, a stepped pin extending downward, including a step portion at a bottom end, and a stepped pin adjuster element. The thermal printhead unit includes an exothermic element array, a supported portion to be hooked on the claw, and a notch portion to be hooked on the step portion. The platen roller unit and the thermal printhead unit include respective positioning elements which engage with each other to restrict a relative movement of the exothermic element array and the platen roller while the cover element is in a closed position.