This paper studies the non-cutting optical fiber transmission tester, which can test the abnormality in the optical fiber core. The basic testing method, in the case of abnormality on the lines, can estimate the loss and other abnormalities by using optical power meter and stabilized laser source of light. The optical measurement wavelength of the system is the wavelength for communication of 1310, 1550nm, and can, in checking the abnormality on the communication lines, measure the optical in order not to affect the existing data communication network with the maximum loss of 0.5dB only. The break down can be diagnosed with the naked eyes by having red laser diode with optical power of 0.5mW(-3dBm) built-in.