The present invention relates to a luggage storage device for a motorcycle. In an embodiment, the invention can allow the capacity of a storage box to be large enough to accommodate long goods therein. In an embodiment, the invention includes a storage box having a front helmet storage portion and a rear helmet storage portion. The front helmet storage portion is disposed under a front seat. The rear helmet storage portion is disposed under a rear seat, which together with the front seat, constitutes a tandem type riding seat. The storage box is disposed under the riding seat so as to extend from under a front end of the riding seat up to near an upper portion of a rear cushion that suspends a rear wheel from a rear portion of a vehicle body frame.