The invention relates to a thickness-measuring method for an organic coating film such as an organic solderability preservative film formed on a metal film. In the method, an absorption spectrum of at least one reference organic coating film formed on a first metal surface is measured and absorption intensity in a predetermined wavelength range is calculated from the absorption spectrum. The thickness of the reference organic coating film is measured by destructive measurement. Then, correlation is defined based upon the absorption intensity and measured thickness of the reference organic coating film. An absorption spectrum of an organic coating film to be measured. Absorption intensity in the predetermined wavelength range is calculated from the absorption spectrum of the organic coating film to be measured, and the thickness of the organic coating film is calculated from the absorption intensity thereof based upon the correlation.