Load cell having a computation unit which computes displacement amount of the diffraction grating

A load cell, comprising:

• an elastic body having a first end portion, a second end portion opposite to said first end portion in one axial direction, and a bridge portion bridging the first and second end portions and defining a hollow space between said first and second end portions so that the first end portion and the second end portion are moveable relative to each other;
• a first optical unit having a light source and a first diffraction grating on which light from said light source is incident, said first optical unit being fixed to said first end portion and arranged in said hollow space;
• a second optical unit having a second diffraction grating on which diffracted light emitted from the first diffraction grating is incident to generate interference light, said second optical unit being fixed to said second end portion and arranged in said hollow space;
• a detector that detects said interference light;
• and
• a computation unit that computes a displacement amount of said second diffraction grating relative to said first diffraction grating on the basis of a signal obtained by said detector so as to detect a displacement amount of the second end potion relative to the first end portion.