A displacement measuring instrument includes a spindle screwed to a stationary sleeve, a thimble, an operation sleeve, and a constant pressure mechanism. The constant pressure mechanism includes: a rotary driving element rotatable together with the operation sleeve; a rotary driven element coupled to the rotary driving element in such a manner as to be rotatable together with the rotary driving element but not to be rotatable when a predetermined or more load is applied to the spindle; and a rotation transmitting mechanism transmitting a rotation of the rotary driven element to the spindle. A male thread portion with the same pitch as that of a male thread portion of the spindle is formed on the outer circumference of the stationary sleeve. A female thread portion is formed on the inner circumference of the thimble and screwed to the male thread portion of the stationary sleeve.