A degree of cure measuring apparatus has: a second optical fiber for emitting light from a tip face thereof; a probe for holding adhesive agent and irradiating the adhesive agent with light while the adhesive agent is in contact with the tip face of the second optical fiber; a detector for detecting light that is reflected from an interface between the tip face of the second optical fiber and the adhesive agent and then returns to the second optical fiber; and a computer for calculating the refractive index of the adhesive agent from the rate of the light amount of the light detected by the detector to the emission light amount from the tip face of the second optical fiber.