PURPOSE: To take-in clean air having no scattered tear mixed therein to inject the same, by providing a valve having an air suction port lower in air inflow resistance than an air jet nozzle to a compression chamber and providing an air suction port to the housing of a tonometer.

CONSTITUTION: A rotary solenoid 1 rotates and a piston 4 advances to a position 4a to inject the air in a compression chamber 6 to the cornea Ec of an eye E to be examined from a nozzle 8 and, when the piston 4 begins to return to the original position, the compression chamber 6 becomes negative pressure. Hereupon, an one-way valve 10 is pushed by atmospheric pressure and overcomes the force of a return spring 11 to open. When the one-way valve 10 is opened, the open air is automatically sucked in the compression chamber 6 from an air suction port 12 lower in air inflow resistance than the nozzle 8. By this method, air reduced in the mixing with a tear can be injected at the time of the next injection of air.