An instrument for measuring tropospheric OH/HO2 radicals by laser-induced fluorescence developed in our laboratory is presented in detail. It is based on FAGE (fluorescence assay by gas expansion) technique and OH is both excited and detected at 308 nm corresponding to its A-X(0,0) band. The alignment of the laser beam, the design of the sample gas inlet, and the devices for the fluorescence detection are optimized so as to reduce the background signal while keeping the OH sensitivity as high as possible. A thermalized position of the expanding gas beam is probed in our system and we did not observe a severe decrease of the HOx sensitivities under humid conditions. An optical fiber is used for delivering the laser light to the fluorescence detection cell mounted outside at a high position. Thus the laser beam alignment is by far simplified and is made highly reproducible, once settled properly. For the calibration, two methods are employed: a system with laser absorption measurements of OH and a system of simultaneous photolysis of H2O and O-2. The calibration factors are compared well within the combined uncertainty. Using the latter system, the conversion efficiency of HO2 to OH by NO addition is measured to be around 90%. The detection limits for OH and HO2 (S/N = 2) are estimated to be 3.3 x 10(6) and 3.6 x 10(6) cm(-)3 at noon, respectively, with an integration time of 1 min. The results of test observations at our institute are also presented.