PURPOSE: To get rid of any troublesomeness for setting an electric load detector and vice versa and any disconnection of harness, to be connected to this detector, due to earth and sand in time of digging by emitting a warning signal at a time when actual lifting load has exceeded the rated load prestored in a controller.

CONSTITUTION: Each distance among respective pins of attachments such as a boom 2, an arm 5, a bucket 7 and each cylinder 4 or the like is prestored in a controller. An operating radius (x) and length L (2) drawn from a boom foot pin 3 to a boom cylinder 4 axis are found in operation by each distance among these pins and the detected value out of each angular sensor of those of boom 2 and arm 5. In addition, in both states of having a lifting load grounded and ungrounded, lifting loadage W (x) is operated by an operation execution command unit. When the calculated lifting loadage W (x) has exceeded the rated load prestored in the controller, a warning signal is emitted.