There is provided an apparatus for measuring a longitudinal modulus of elasticity of a fiber used for composite material, including a holder for supporting a single fiber at its one end in a cantilever fashion, a heater for heating the single fiber, a measurement device for measuring deflection of the single fiber, and a calculator for calculating a longitudinal modulus of elasticity of the single fiber in accordance with a deflection measured by the measurement device, provided that the single fiber is equivalent to a beam receiving uniformly distributed load thereon based on a dead weight thereof. For instance, the heater includes a closed container having a transparent window through which the deflection of the single fiber is measured The measurement device is a laser displacement gauge or an optical displacement gauge. The apparatus makes it possible to readily and rapidly measure a longitudinal modulus of elasticity of a single fiber in various temperatures including room temperature by using only a single fiber.