A frequency resolution for measuring transmission characteristics of a device under test is increased. With a measuring device including a first terahertz light generator that generates incident light, a second terahertz light generator that generates reference light having an optical frequency f1−f2−fIF different from an optical frequency f1−f2 of the incident light by a constant difference frequency fIF, a terahertz light detector which outputs an light detection signal having the difference frequency fIF based on response light obtained by making the incident light incident to an optical fiber and the reference light, and a network analyzer that receives the light detection signal, thereby measuring characteristics of the optical fiber, a spectrum of the incident light (terahertz light) incident to the optical fiber includes the carrier frequency (f1−f2), but does not include sideband frequencies (f1−f2±fIF). It is thus possible to reduce the effective spectrum width of the incident light. As a result, the frequency resolution increases in the measurement of the transmission characteristics of the optical fiber.