An evanescent catheter system having a tip portion of an optical fiber capable of securely measuring fluorescence intensity or fluorescence spectrum of a substance around a catheter tip, without being hindered by optical absorption of hemoglobin in erythrocytes in blood, and being capable of calculating an existing amount of a fluorescence-emitting substance existing in an evanescent light generating part. A protecting layer 22 and a cladding 23 are removed from an upper semicircular part of a core 24 at a tip of a catheter 21 to expose the core 24. When excitation light 29 is incident, evanescent light 25 is generated on the upper semicircular part of the core 24 and a fluorescent substance generates emits fluorescence 30. An interference filter 26 filters out the excitation light and only the fluorescence 30 reaches the photodiode 28, permitting a measuring device to measure an existing amount of the fluorescent substance.