To exactly monitor operation load of a soil and sand conveying pump by discriminating the operation period of a piston for soil and sand conveying and a movable cylinder in each process, and detecting to indicate oil pressure in the operation period after a time required for one cycle is previously calculated in response to detected oil pressure behavior and respective time distribution of each process occupied during the time required for one cycle on the basis of a pressure oil flow rate necessary for one cycle and the pressure oil flow rate respectively necessary for each process.

Detected signals from oil pressure detectors 45, 46 are successively stored to a memory. When the instruction of a process from an operation switch 52 is input, numerical data of a one-cycle part just before the inside of the memory are used to calculate a time necessary for one cycle. Thereafter the time necessary for one cycle is calculated by using total necessary pressure oil flow rates and a pressure oil flow rate classified by a process necessary for each process stored in the memory. The operation times of a cylinder and a piston are discriminated in each process, and oil pressure is detected in the operation time to be indicated on a display 49.