A telecentric lens system (3) including a front lens unit (G1), a rear lens unit (G2) which is disposed so that the front focal point of the rear lens unit coincides with the back focal point of the front lens unit, and a diaphragm mechanism (31) which is disposed at the location where the back focal point of the front lens unit (G1) and the front focal point of the rear lens unit (G2) coincide, and which comprises a numerical aperture changer (33,34) for changing the numerical aperture of the diaphragm mechanism (31). By changing the numerical aperture, the telecentric lens system (3) can be used in a wide range of applications from observations requiring a large focal depth to observations requiring high resolution. The invention provides the telecentric lens system (3) which can be used in observations for a wide range of applications and an image measuring device (1) including the telecentric lens system (3).