To provide an inexpensive analog optical fiber sensor of a simple constitution.

Changes in a liquid level are converted into mechanical displacements by a conversion part 1. The mechanical displacements are converted into optical intensity by a mechanism part 2 of a movable optical system to measure the optical intensity of measuring light. The mechanical displacements is measured by the measuring light by making reference to previously related reference light by a reference part 3 of a fixed optical system. In the mechanism part 2, emergent light of an incident and emergent part 2a for measurement of an optical fiber 4 for measurement is reflected by a vertically moving reflecting film 2b for measurement, and reflected light is made to return as measuring light from the incident and emergent part for measurement to the optical fiber for measurement. In the reference part 3, emergent light of an incident and emergent part 3a for reference of an optical fiber 5 for reference is reflected by a fixed reflecting film 3b for reference, and reflected light is made to return as reference light from the incident and emergent part for reference to the optical fiber for reference.