Abstract: A tonometer having a construction whereby thermal expansion or contraction of the material forming the tonometer casing will have substantially no influence upon the precision of measurement of the instrument. The tonometer comprises a casing, a bellows mounted in a bottom portion of the casing, a rod resting on the bellows adapted to be axially shifted as the bellows expands and contracts, a base plate mounted above the bellows in the main portion of the casing, and an amplifying mechanism including a pointer mounted on the base plate in operative association with the rod for swinging the pointer. The base plate includes at least one leg forming an extension thereof which is fixed with respect to the casing so that the leg or legs constitute the sole means by which the base plate is fixedly mounted with respect to the casing. The rod rests on the bellows at a position closely adjacent to a position at which the base plate is fixed with respect to the casing.