  We measured the thickness of an insensitive surface layer of a PIN
photodiode, Hamamatsu S3590-06, used in the Tokyo Axion Helioscope. We made
alpha-particles impinge on the PIN photodiode in various incidence angles and
measured the pulse height to estimate the thickness of the insensitive surface
layer. This measurement showed its thickness was $0.31 \pm 0.02 \mu m$ on the
assumption that the insensitive layer consisted of Si. We calculated the peak
detection efficiency for low energy x-rays in consideration of the insensitive
layer and escape of x-rays and Auger electrons. This result showed the
efficiency for 4-10keV x-rays was more than 95%.
