A planing heads positioning device to be used with an automatic planing machine for planing sides of a piece of lumber moving along a travelling course is provided. The device includes a main frame and a planing assembly including first and second planing head arranged on opposite sides of the travelling course. The planning assembly further includes first and second supports respectively supporting the first and second planing heads. The first support is slidably mounted to the main frame, and the second support is operatively connected to the first support. The device also includes a primary positioning mechanism connected to the first support to position the planing assembly relative to the main frame; and a secondary positioning mechanism operatively connected to the supports to modify the planing width.