A serial-to-parallel converting apparatus, comprising: an input plate connected to positive electrodes and negative electrodes of a plurality of battery cells, respectively, wherein the input plate has circular shape and is divided into equal areas; an output plate spaced apart from the input plate and connected to an external terminal; a rotary plate rotatably interposed between the input plate and the output plate to electrically connect the input plate and the output plate, the rotary plate being configured to convert serial connection and parallel connection of the plurality of battery cells by rotating; and a case configured to accommodate the input plate, the output plate and the rotary plate; wherein the input plate, the rotary plate and the output plate are stacked on each other, and the input plate is completely fixed to the case and not able to move linearly or rotate; wherein the output plate is configured to be movable toward the input plate or away from the input plate; wherein the output plate is in contact with the rotary plate to move toward the input plate or away from the input plate together with the rotary plate; wherein a fixing protrusion or a fixing groove is formed at the output plate and the case, respectively, and the fixing protrusion and the fixing groove are coupled to restrict rotation of the output plate; wherein an insulation region is formed in at least a portion of the rotary plate; wherein the rotary plate has a simultaneous connection region to which a positive electrode of one cell and a negative electrode of another cell are simultaneously connected by the rotation of the rotary plate. an input plate connected to positive electrodes and negative electrodes of a plurality of battery cells, respectively, wherein the input plate has circular shape and is divided into equal areas; an output plate spaced apart from the input plate and connected to an external terminal; a rotary plate rotatably interposed between the input plate and the output plate to electrically connect the input plate and the output plate, the rotary plate being configured to convert serial connection and parallel connection of the plurality of battery cells by rotating; and a case configured to accommodate the input plate, the output plate and the rotary plate; wherein the input plate, the rotary plate and the output plate are stacked on each other, and the input plate is completely fixed to the case and not able to move linearly or rotate; wherein the output plate is configured to be movable toward the input plate or away from the input plate; wherein the output plate is in contact with the rotary plate to move toward the input plate or away from the input plate together with the rotary plate; wherein a fixing protrusion or a fixing groove is formed at the output plate and the case, respectively, and the fixing protrusion and the fixing groove are coupled to restrict rotation of the output plate; wherein an insulation region is formed in at least a portion of the rotary plate; wherein the rotary plate has a simultaneous connection region to which a positive electrode of one cell and a negative electrode of another cell are simultaneously connected by the rotation of the rotary plate.