A method for adjusting the comfort level of a helmet includes 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 showing the space information on a display part by superimposing the additional images on i) a shape image showing the inner surface shape data or ii) the shape image showing the head shape data and the inner surface shape data.