1. A functional cap comprising: a holder 20 having a fitting groove 21 formed along the inner edges thereof of the front side thereof, holder wings 22 formed along the outside surface of the fitting groove 21 , and a back plate 23 formed on the bottom surface thereof, the back plate 23 having wing grooves 27 formed opened outwardly by a given depth, fitting wings 26 formed on the outside of the wing grooves 27 , and a plurality of ventilating holes 24 formed thereon for allowing the heat generated from a wearer's skin at the inside of the cap body 10 to be reduced by means of with external fresh air introduced therethrough; a cap body 10 having a cap hole 12 formed at a given position thereof, the cap hole 12 having a cap section 13 adapted to be fit into the wing grooves 27 ; and a plate 30 detachably mounted to the holder 20 , the plate 30 having a specific picture, letter, or character printed on one surface thereof.