PURPOSE: To provide quick diagnosis for failure in an oil-hydraulic pump by emitting a failure signal about the hydraulic pump when the absolute value of the difference between a command value, which is to displace a substitutional capacity varying mechanism (swash plate) in a certain specific amount, and the displacement amount of the swash plate displaced in conformity to said command value has exceeded a specific tolerable level.

CONSTITUTION: When a control lever 9 is operated, a swash plate 6a (substitutional capacity varying mechanism) is driven in accordance with the deviation of the operation signal X from the displacement signal Y. This operation signal X is fed into an adding circuit 12a of a failure judgement circuit 11, where an addition is made to a predetermined tolerable value Δ, and the sum X+Δ obtained is compared with the Y at a comparator 13a. If Y>(X+Δ), the compartor 13a emits a high level output I as a signal to indicate that there is a failure in the oil-hydraulic pump. The signal X is fed into another adding circuit 12b, in the same manner, where a subtraction is made with said tolerable value Δ, and the difference (X-Δ) obtained is compared with the signal Y at a comparator 13b. If Y<(X-Δ), the comparator 13b emits an output I. In case the absolute value of the difference between the signals Y and X exceeds the tolerable value Δ, a diode 15 emits a failure signal immediately.