The qualities of fiber Bragg grating (FBG) strain data acquired in the long-term structural health monitoring of an existing concrete bridge were evaluated in this study. The possible sources of errors in the acquired data were first considered on the basis of the principle of the FBG sensor and the details of the installation of system components: FBGs, cables, epoxy, FBG interrogator, and data acquisition system. The data reliability was first evaluated by a defined measure: the availability ratio of FBG peak wavelengths throughout the period of 2012–2014. It was then said that the high reliabilities of the FBG interrogator and data acquisition system were ensured; however, reliability reduction was observed in some FBGs. The stability was also evaluated on the basis of the accuracy and sensitivity. A characteristic disturbance in the data accuracy, which is the standard deviation of the acquired FBG peak wavelength, was recognized only in specific FBGs. Moreover, the attachment condition issues in some FBGs were also recognized by sensitivity analyses. The results and considerations in this paper indicated the importance of those data quality analyses for the appropriate use of the long-term data in structural condition assessment throughout the life of existing civil infrastructures.