Provided is a device capable of determining whether or not cancer has metastasized to a sentinel lymph node, such as the axillary lymph node, thickly covered with fat, safely and in a simple manner without confirmation by biopsy or extirpation. Fluorescence generated in a sentinel lymph node upon the excitation of protoporphyrin (a photosensitizing substance) derived from 5-ALA administered to a subject can be detected by passing a hollow tube, into which a small-diameter optical fiber for launching light and the small-diameter optical fiber for receiving light are inserted, through the lymph vessel to identify the presence or absence of the metastasis of cancer cells even before the surgery of cancer or during prognosis.
