An abnormality determination device for a rotation sensor that detects rotation of a rotational body, wherein the rotation sensor includes a first sensor portion (42, 43) and a second senor portion (42, 43), each outputting a pulse signal having an output width corresponding to one half of a predetermined angle whenever the rotational body is rotated by the predetermined angle, the first and second sensor portions output signals having phases shifted from each other, and the rotation sensor outputs a pulse signal when a condition is satisfied where the output signal of the second sensor portion shifts in a state in which the rotation sensor does not output a pulse signal and the output signal of the first sensor portion is a predetermined value, outputs a pulse signal having a first time width when a shifting direction of the output signal of the second sensor portion is in a first direction, and outputs a pulse signal having a second time width that is longer than the first time width when the shifting direction of the output signal of the second sensor portion is in a second direction that is opposite to the first direction, the abnormality determination device characterized by: a calculation unit that calculates a time interval of output initiation times of the pulse signals; and a determination unit that determines that the rotation sensor is abnormal, wherein the rotational body is an output shaft of an internal combustion engine; the rotation sensor outputs a pulse signal having the first time width as a signal indicating normal rotation of the output shaft and outputs a pulse signal having the second time width as a signal indicating reverse rotation of the output shaft; the rotation sensor includes a signal rotor having projections (41 a), which are formed on its outer circumference at predetermined angular intervals, and a missing-teeth portion (41 b), which is formed by removing some of the projections, wherein the signal rotor is attached to an integrally rotatable with the output shaft; each of the first sensor portion and the second sensor portion outputs a pulse signal whenever a projection of the signal rotor passes by; the abnormality determination device further comprises a missing-teeth detection unit that detects passage of the missing-teeth portion when a difference between two of the time intervals consecutively calculated by the calculation unit is greater than a determination value; and the determination unit then determines that the rotation sensor has an abnormality when the number of time that the missing-teeth detection unit detects passage of the missing-teeth portions is greater than or equal to a threshold value that is greater than the number of the missing-teeth portion during a period in which the pulse signal is output from the rotation sensor for a number of times indicating that the output shaft has rotated once. a calculation unit that calculates a time interval of output initiation times of the pulse signals; and a determination unit that determines that the rotation sensor is abnormal, wherein the rotational body is an output shaft of an internal combustion engine; the rotation sensor outputs a pulse signal having the first time width as a signal indicating normal rotation of the output shaft and outputs a pulse signal having the second time width as a signal indicating reverse rotation of the output shaft; the rotation sensor includes a signal rotor having projections (41 a), which are formed on its outer circumference at predetermined angular intervals, and a missing-teeth portion (41 b), which is formed by removing some of the projections, wherein the signal rotor is attached to an integrally rotatable with the output shaft; each of the first sensor portion and the second sensor portion outputs a pulse signal whenever a projection of the signal rotor passes by; the abnormality determination device further comprises a missing-teeth detection unit that detects passage of the missing-teeth portion when a difference between two of the time intervals consecutively calculated by the calculation unit is greater than a determination value; and the determination unit then determines that the rotation sensor has an abnormality when the number of time that the missing-teeth detection unit detects passage of the missing-teeth portions is greater than or equal to a threshold value that is greater than the number of the missing-teeth portion during a period in which the pulse signal is output from the rotation sensor for a number of times indicating that the output shaft has rotated once.