Laser-produced line plasmas were used as a broadband source of soft X-rays and a stigmatic grazing incidence grating spectrometer to measure the normal incidence reflectance of concave multilayer mirrors in the 20 ~ 40 nm spectral region. The measurements of the reflectance made on a C/Al multilayer show a little lower than the calculated results and indicate a normal incidence peak reflectance of about 22% at 28.5 nm.