the equation 7 is calculated by the addition circuit a 2 the value u03b8 is obtained by inverting the result of the calculation by the inverter in 1 this value is integrated twice to obtain the value however since integration error is caused in the integration operation an arrangement has been made in the invention to correct the integration error furthermore it goes without saying that correction is made to minimize the above described noise component then the calculation of the following equation 10 is carried out equ5 therefore u03b8 is independent of k 1 and k 2 in addition the accumulation error in the integration can be substantially minimized the value 1 s is the transfer function of the integrator with respect to the noise component e b nx the following calculation is carried out equ6 the transfer function of the equation 13 can be sufficiently reduced by suitably selecting the values k 1 and k 2 that is the effect of the noise component can be sufficiently reduced accordingly the signal e d excellent in response characteristic and sufficiently free from the effect of the acceleration caused at random can be obtained a cylinder stroke detector 23 is juxtaposed with the blade cylinder 5 to detect the stroke of the blade cylinder 5 thereby to output a stroke signal e s an inclinometer 24 is provided at a predetermined position on the rear surface of the blade 4 to detect the tilt angle of the blade 4 to output a tilt angle signal e c an arithmetic circuit 26 receives the inclination.