To increase the moving range of a boom tip as much as possible, and ensure smooth crane operation, while sufficiently preventing a crane from turning over backward.

The backward turnover prevention device includes: a boom length detection means; a boom angle detection means; and a control means to perform control to regulate the boom length reducing operation or the boom raising operation when the crane is likely to turn over backward. The control region by the control means includes a first region from the minimum boom angle to a limit boom angle θa to regulate the boom raising motion in order to secure predetermined backward stability when the expansion length of the boom is the shortest, and a second region from the limit boom angle to the maximum boom angle. In the second region, the limit length La of the boom is set so that the center-of-gravity positions G11, G1n, Glm of the boom are, at all the boom angles, aligned with the center-of-gravity position G2 of the boom at the limit boom angle on a vertical line V, and the boom length reducing operation is regulated when the boom length becomes shorter than the limit length.