An optical transmission line monitoring apparatus comprising: a measuring device is adapted to introduce an optical pulse along a longitudinal direction of an optical fiber constituting an optical transmission line, and is adapted to measure the optical intensity of returning light obtained by the incidence and a distance based on a time when the returning light is received; a reference data storage device is adapted to store a reference loss amount or a reference reflection attenuation amount beforehand; a detecting device that is connected to the measuring device; an alarm outputting device (13, 13a, and 13b) is adapted to create alarm data based on the detection data created by the detecting device, and is adapted to display the created alarm data on a screen; an adding device; characterized in that: the detecting device is adapted to calculate a loss amount or a reflection attenuation amount by performing an analysis of the optical intensity obtained from the measuring device, the detecting device detecting a distance when the calculated loss amount is of poorer quality than the reference loss amount stored in the reference data storage device, creating detection data containing the loss amount at the time of detection, and storing the created detection data in a detection data storage device, or the detecting device detecting a distance when the calculated reflection attenuation amount is of poorer quality than the reference reflection attenuation amount stored in the reference data storage device, creating detection data containing the reflection attenuation amount at the time of detection, and storing the created detection data in a detection data storage device; an inputting device, when manipulated by a user, is adapted to input selection command data that indicates that the alarm data displayed on the screen is selected as alarm data that will not be output; and an adding device (12, 12a, and 12b) is adapted to add, to the detection data stored in the detection data storage device corresponding to the alarm data of the selection command data input by the inputting device, output removal data indicating that it is not a target for outputting by the alarm outputting device (13, 13a, and 13b); the alarm outputting device (13, 13a, and 13b) is adapted to display alarm data displayed on the screen, corresponding to the detection data that the adding device (12, 12a, and 12b) is adapted to add the output removal data to, such that it can be distinguished from other alarm data. a measuring device is adapted to introduce an optical pulse along a longitudinal direction of an optical fiber constituting an optical transmission line, and is adapted to measure the optical intensity of returning light obtained by the incidence and a distance based on a time when the returning light is received; a reference data storage device is adapted to store a reference loss amount or a reference reflection attenuation amount beforehand; a detecting device that is connected to the measuring device; an alarm outputting device (13, 13a, and 13b) is adapted to create alarm data based on the detection data created by the detecting device, and is adapted to display the created alarm data on a screen; an adding device; characterized in that: the detecting device is adapted to calculate a loss amount or a reflection attenuation amount by performing an analysis of the optical intensity obtained from the measuring device, the detecting device detecting a distance when the calculated loss amount is of poorer quality than the reference loss amount stored in the reference data storage device, creating detection data containing the loss amount at the time of detection, and storing the created detection data in a detection data storage device, or the detecting device detecting a distance when the calculated reflection attenuation amount is of poorer quality than the reference reflection attenuation amount stored in the reference data storage device, creating detection data containing the reflection attenuation amount at the time of detection, and storing the created detection data in a detection data storage device; an inputting device, when manipulated by a user, is adapted to input selection command data that indicates that the alarm data displayed on the screen is selected as alarm data that will not be output; and an adding device (12, 12a, and 12b) is adapted to add, to the detection data stored in the detection data storage device corresponding to the alarm data of the selection command data input by the inputting device, output removal data indicating that it is not a target for outputting by the alarm outputting device (13, 13a, and 13b); the alarm outputting device (13, 13a, and 13b) is adapted to display alarm data displayed on the screen, corresponding to the detection data that the adding device (12, 12a, and 12b) is adapted to add the output removal data to, such that it can be distinguished from other alarm data.