The present invention provides a diffraction grating element that allows the
temperature control mechanism to be dispensed with or simplified. The diffraction
grating element of the present invention comprises a transparent plate having a
first surface and a second surface that are substantially parallel with one another;
and a diffraction grating which is formed on a first surface side with respect
to the second surface and is substantially parallel with the first surface. At
any temperature within a temperature range -;20 C. to +80 C., the sum
of the rate of change in the period per unit length of the diffraction grating
with respect to a temperature change, and the temperature coefficient of the refractive
index of a medium that surrounds the diffraction grating element is 0.
