The invention relates to a moveable working device, particularly an auto concrete pump, comprising an undercarriage (10) provided with two front and two rear supporting extension arms (22,24) which can be extended from a travel position into at least one support position, which can be supported on a foundation (36) and which are respectively provided with a supporting leg (28), also provided with a respective measuring member (38) for determining the support load acting upon the supporting legs (28). If the telescopic supporting legs (28) with the telescopic member (30) fixed to the extension arm are coupled to a support leg casing by means of a coupling bolt (32), it is possible to configure the coupling bolt as a measuring member (38) in order to determine the support load, using particularly simple means. The measuring members can be incorporated into a stability monitoring device comprising a computer-assisted evaluation electronics system (68,74). A software routine is provided for stability control in order to determine a stability figure (S) which is determined on the basis of the quotients of the overall sum of the support load measurement values of all supporting legs (28) and a partial sum of the support load measurement values of the two supporting legs (28) which are momentarily subjected to a support load. An alarm routine is also provided in order to trigger an alarm state if said values fall below a given stability threshold value (S1, S2, S3).