A magnetic sensor device includes a magnetic sensor, and a soft magnetic structure disposed near the magnetic sensor. The magnetic sensor and the soft magnetic structure are configured so that when an external magnetic field including a detection-target magnetic field is applied to the magnetic sensor, the external magnetic field is also applied to the soft magnetic structure, and when the soft magnetic structure has a magnetization, a magnetic field based on the magnetization of the soft magnetic structure is applied to the magnetic sensor. The soft magnetic structure has a stripe domain structure.