An optical time domain reflectometer includes: a storage unit that stores in advance a fourth backscattered light level PZ(W2) obtained by adding a difference ΔPR between a second backscattered light level PR(W1) which is measured using a first pulse width W1 and a third backscattered light level PR(W2) which is measured using a second pulse width W2 to a first backscattered light level PZ(W1) at a connection point DD to a measurement target optical fiber which is measured using the first pulse width so as to be associated with the second pulse width; and an arithmetic processing unit that reads the fourth backscattered light level PZ(W2) at the second pulse width corresponding to a pulse width of an optical pulse output from a light source to the measurement target optical fiber from the storage unit and calculates a transmission loss LT in the measurement target optical fiber.