A method for adjusting a comfort level of a helmet, the method comprising the steps of: acquiring head shape data by measuring a three-dimensional shape of a head of a wearer of a helmet; inputting inner surface shape data showing a three-dimensional shape of a helmet inner surface; acquiring space information showing a space between the head and the helmet inner surface on the basis of the head shape data and the inner surface shape data; and displaying additional images (G4; G5) showing the space information on a display part by superimposing the additional images on i) a shape image corresponding to the inner surface shape data or ii) shape images (G1; G2) corresponding to the head shape data and the inner surface shape data, characterised in that the displaying step displays, as the additional images (G4; G5), border lines between a plurality of regions (R2a to R2i, R3a to R3f) on an attachment site where additional pads (38, 38a, 38b) includes a first additional pad having a reference shape and second additional pad and third additional pad layered on the first additional pad having a shape that is cut from the reference shape along the border lines of the the additional images (G4; G5), and the displaying step displays the additional image (G4; G5) on the display part by superimposing it on the attachment site in the shape image, specifying a form of the additional pad to be added to the helmet inner surface (la) on the basis of the additional image displayed on the display part; and attaching the additional pad corresponding to the form that is specified in the specifying step to a predetermined attachment site in the helmet inner surface. acquiring head shape data by measuring a three-dimensional shape of a head of a wearer of a helmet; inputting inner surface shape data showing a three-dimensional shape of a helmet inner surface; acquiring space information showing a space between the head and the helmet inner surface on the basis of the head shape data and the inner surface shape data; and displaying additional images (G4; G5) showing the space information on a display part by superimposing the additional images on i) a shape image corresponding to the inner surface shape data or ii) shape images (G1; G2) corresponding to the head shape data and the inner surface shape data, characterised in that the displaying step displays, as the additional images (G4; G5), border lines between a plurality of regions (R2a to R2i, R3a to R3f) on an attachment site where additional pads (38, 38a, 38b) includes a first additional pad having a reference shape and second additional pad and third additional pad layered on the first additional pad having a shape that is cut from the reference shape along the border lines of the the additional images (G4; G5), and the displaying step displays the additional image (G4; G5) on the display part by superimposing it on the attachment site in the shape image, specifying a form of the additional pad to be added to the helmet inner surface (la) on the basis of the additional image displayed on the display part; and attaching the additional pad corresponding to the form that is specified in the specifying step to a predetermined attachment site in the helmet inner surface.