A method performed by a blind-spot monitoring device for monitoring at least one blind spot of a cycle using a smart helmet to be used for a rider of the cycle, wherein the at least one blind spot of the cycle is one or more preset areas set as areas where the rider cannot perceive visually depending on a traveling direction of the cycle and the smart helmet having at least one camera or at least one radar installed thereon, the method comprising steps of: (a) if at least one video image of one or more 1-st blind spots corresponding to the smart helmet worn by the rider is acquired from the at least one camera or radar, detecting by an object detector one or more objects on the video image and confirming one or more 1-st objects located in the one or more 1-st blind spots among the detected objects; and (b) determining a smart helmet orientation and a cycle traveling direction by using sensor information acquired from at least part of a GPS sensor, an acceleration sensor, and a geomagnetic sensor installed on the smart helmet, confirming one or more 2-nd objects, among the one or more 1-st objects, located in one or more 2-nd blind spots corresponding to the blind spots of the cycle by referring to the smart helmet orientation and the cycle traveling direction, wherein confirming one or more 2-nd objects, among the one or more 1-st objects, is performed by: calculating at least one angular difference between the smart helmet orientation and the cycle traveling direction converting one or more 1-st locations of the one or more 1-st objects into one or more relative locations corresponding to the cycle traveling direction by using the at least one angular difference, determining at least part of the one or more 1-st objects, corresponding to at least part of the relative locations matching the one or more 2-nd blind spots, as the 2-nd objects, and the method further comprising, subsequent to confirming, the steps of displaying the 2-nd objects via a head-up display installed on the smart helmet or sounding an alarm representing that the 2-nd objects are located in the one or more 2-nd blind spots via at least one speaker installed on the smart helmet (a) if at least one video image of one or more 1-st blind spots corresponding to the smart helmet worn by the rider is acquired from the at least one camera or radar, detecting by an object detector one or more objects on the video image and confirming one or more 1-st objects located in the one or more 1-st blind spots among the detected objects; and (b) determining a smart helmet orientation and a cycle traveling direction by using sensor information acquired from at least part of a GPS sensor, an acceleration sensor, and a geomagnetic sensor installed on the smart helmet, confirming one or more 2-nd objects, among the one or more 1-st objects, located in one or more 2-nd blind spots corresponding to the blind spots of the cycle by referring to the smart helmet orientation and the cycle traveling direction, wherein confirming one or more 2-nd objects, among the one or more 1-st objects, is performed by: calculating at least one angular difference between the smart helmet orientation and the cycle traveling direction converting one or more 1-st locations of the one or more 1-st objects into one or more relative locations corresponding to the cycle traveling direction by using the at least one angular difference, determining at least part of the one or more 1-st objects, corresponding to at least part of the relative locations matching the one or more 2-nd blind spots, as the 2-nd objects, and the method further comprising, subsequent to confirming, the steps of displaying the 2-nd objects via a head-up display installed on the smart helmet or sounding an alarm representing that the 2-nd objects are located in the one or more 2-nd blind spots via at least one speaker installed on the smart helmet calculating at least one angular difference between the smart helmet orientation and the cycle traveling direction converting one or more 1-st locations of the one or more 1-st objects into one or more relative locations corresponding to the cycle traveling direction by using the at least one angular difference, determining at least part of the one or more 1-st objects, corresponding to at least part of the relative locations matching the one or more 2-nd blind spots, as the 2-nd objects, and the method further comprising, subsequent to confirming, the steps of displaying the 2-nd objects via a head-up display installed on the smart helmet or sounding an alarm representing that the 2-nd objects are located in the one or more 2-nd blind spots via at least one speaker installed on the smart helmet