1. A disk apparatus for a wheel of a motorized vehicle, having a detachable/attachable structure from/to a side of the wheel, the disk apparatus comprising: a fixing plate provided at both side surfaces thereof with coil units (110, 110'), which are radially disposed, and formed at a center thereof with a through-hole so as to be coupled to a wheel shaft of the wheel; and a pair of ring-shaped rotational plates (200, 200') facing each other, having a ring shape and formed at side surfaces thereof with magnet units (210, 210') disposed radially corresponding to the coil units (110, 110') of the fixing plate, wherein the fixing plate includes a main fixing plate having a disk shape and a through-hole at the center thereof, and a ring-shaped fixing plate having a circular-plate shape and coil units (110, 110') radially disposed on both side surfaces of the ring-shaped fixing plate, the main fixing plate coupled to the wheel shaft through the through-hole, and the ring-shaped fixing plate coupled to one side surface of the main fixing plate through a coupling unit to form a step difference, wherein the ring-shaped rotational plates (200, 200') are coupled through a coupling unit to a rim coupled to the wheel shaft through a bearing and a hub, such that the coil units (110, 110') and the magnet units (210, 210') correspond to each other while being spaced apart from each other at a same interval on both side surfaces of the ring-shaped fixing plate, whereby, when the fixing plate and the ring-shaped rotational plates (200, 200') are installed to a rim-type wheel having a spoke connected to the hub, the main fixing plate is first coupled to the wheel shaft and one ring-shaped rotational plate is coupled to the spoke, which is inside the wheel, at one side of the ring-shaped fixing plate through the coupling unit, the other ring-shaped rotational plate is coupled to a side surface of the rim at an opposite side of the ring-shaped fixing plate through the coupling unit, such that the disk apparatus is detachably attached to an existing wheel. a fixing plate provided at both side surfaces thereof with coil units (110, 110'), which are radially disposed, and formed at a center thereof with a through-hole so as to be coupled to a wheel shaft of the wheel; and a pair of ring-shaped rotational plates (200, 200') facing each other, having a ring shape and formed at side surfaces thereof with magnet units (210, 210') disposed radially corresponding to the coil units (110, 110') of the fixing plate, wherein the fixing plate includes a main fixing plate having a disk shape and a through-hole at the center thereof, and a ring-shaped fixing plate having a circular-plate shape and coil units (110, 110') radially disposed on both side surfaces of the ring-shaped fixing plate, the main fixing plate coupled to the wheel shaft through the through-hole, and the ring-shaped fixing plate coupled to one side surface of the main fixing plate through a coupling unit to form a step difference, wherein the ring-shaped rotational plates (200, 200') are coupled through a coupling unit to a rim coupled to the wheel shaft through a bearing and a hub, such that the coil units (110, 110') and the magnet units (210, 210') correspond to each other while being spaced apart from each other at a same interval on both side surfaces of the ring-shaped fixing plate, whereby, when the fixing plate and the ring-shaped rotational plates (200, 200') are installed to a rim-type wheel having a spoke connected to the hub, the main fixing plate is first coupled to the wheel shaft and one ring-shaped rotational plate is coupled to the spoke, which is inside the wheel, at one side of the ring-shaped fixing plate through the coupling unit, the other ring-shaped rotational plate is coupled to a side surface of the rim at an opposite side of the ring-shaped fixing plate through the coupling unit, such that the disk apparatus is detachably attached to an existing wheel.