An apparatus and method for excavating tunnels with a shield and slurry while detecting and preventing excess excavation in high water content soft ground wherein, in each of feeding system of slurry to tunnel face and discharging system of a mixture of slurry and excavated soil or the like as well as gushing water from the tunnel face out of working area, the respective volumes of the slurry and mixture passing through the respective systems are measured in positions close to the tunnel face, the volume of the excavated soil or the like is determined with the difference between both volumes while the amount of propelling a tunnelling machine is determined as an excavated space volume of the tunnel, the respective values of the excavated soil volume and excavated space volume are compared with each other, whereby the amount of propelling the machine, fed amount of the slurry or discharging amount of the mixture can be properly adjusted so that the difference between said values will be made substantially zero.