Provided are a monitoring device, a monitoring method, an optical amplifier, and an optical transmission system that are adapted for an increase in the number of cores in a multi-core optical fiber transmission path, and that are suitable for crosstalk monitoring. The monitoring device monitors a multi-core optical fiber transmission path comprising a plurality of use core and at least one or more non-use cores. The monitoring device comprises: an applying means for applying dithering to signal light propagating in the use cores; a monitoring means for monitoring the power of the non-use cores; and a separating means for separating a monitoring result from the monitoring means into power components from the plurality of use cores.

An optical communication system includes: an optical fiber including a plurality of cores; a first optical communication device provided with an optical transmission unit and an optical reception unit each connected to one end of one of the cores on an interface surface; and a second optical communication device provided with an optical transmission unit and an optical reception unit each connected to another end of any one of the cores on an interface surface, in which one of the connection ends of the first optical communication device and of the second optical communication device being in a line-symmetric positional relationship with respect to a vertical line to a horizontal plane of when the first optical communication device and the second optical communication device are installed on the horizontal plane such that the interface surfaces of the first optical communication device and the second optical communication device face a front.