Provided is a crank angle detection device that includes an ACG rotor supported by an end of a crank shaft and configured to rotate integrally with the crank shaft, a metal-plate-made pulsar ring including a ring-shaped plate portion and a detected portion that includes a plurality of convex portions formed at an outer circumference of the ring-shaped plate portion, and being configured to rotate integrally with the ACG rotor with the ring-shaped plate portion fixed to the ACG rotor, and a magnetic sensor arranged at an outer circumference of the pulsar ring and configured to detect the detected portion. Such a crank angle detection device is capable of detecting a crank angle with high accuracy.