Cross member of vehicle body and method of manufacturing cross member

A cross member comprising a plate member with a predetermined sectional shape, the plate member constituting a portion of a vehicle body frame, extending in a vehicle width direction, and being formed of metal, wherein:

• the plate member is provided with a level difference portion on a first surface of the plate member, and a cut mark portion on a second surface of the plate member, the second surface being on the opposite side of the first surface of the plate member;
• and
• the plate member is provided with a thick plate portion on a first side of the plate member in a direction in which the plate member extends, and a thin plate portion on a second side of the plate member in the direction in which the plate member extends, with the level difference portion interposed between the first and second sides.