A platen moving mechanism has a main body frame having a first recess, a head that performs processing on a sheet material, and a platen roller having a shaft portion that is inserted in the first recess of the main body frame while the sheet material is fed in a preselected feeding direction between the platen roller and the head during processing of the sheet material. A platen frame supports the platen roller and is mounted to undergo pivotal movement between an open state and a closed state. The platen frame has a second recess that retains the shaft portion of the platen roller during processing of the sheet material and from which the shaft portion can be completely removed in the open state of the platen frame. A lock arm is mounted to undergo pivotal movement to support the platen roller in the closed state of the platen frame. The lock arm has a third recess in which the shaft portion of the platen roller is inserted in the closed state of the platen frame. During the processing of the sheet material, the platen frame remains in the closed state while the shaft portion of the platen roller is securely retained in the second recess of the platen frame and inserted in the first recess of the main body frame and the third recess of the lock arm.
