To provide a crack investigation method and a crack investigation device for a steel structure capable of obtaining quickly an accurate inspection data, capable of enhancing efficiency of investigation work in a field, and quality of the inspection data, easy to grasp the inspection data, and capable of facilitating investigation and evaluation.

An investigation objective part is imaged by a camera 2 faced thereto, an appearance image is stored, a slit light projector 3 is installed for the investigation objective part to bring an optical axis 3a thereof into a prescribed inclination angle θ with respect to the view field center 2a of the camera 2, slit light is projected from the slit light projector 3 to be imaged with the camera 2, a cross-sectional shape image of the investigation objective part is recorded, a magnetic sheet 1 containing magnetic powder and covered with a transparent sheet is bonded onto a surface of the investigation objective part, the investigation objective part is magnetized thereafter to change the orientation of the magnetic powder, and a condition of light intensity generated on a magnetic sheet 1 surface is imaged by the camera 2 to recorded a surface crack image.