A grit chamber for a wastewater treatment plant. A vertical floodgate is installed on a partitioning wall and is driven in a vertical direction to be opened downwardly from a top toward a bottom. A floodgate driving motor is coupled to one end of the vertical floodgate to directly drive the vertical floodgate. An inflow amount measurement sensor functions to stepwisely measure a flow amount of wastewater which passes through a wastewater tank. A control section is connected to the floodgate driving motor and the inflow amount measurement sensor to control the floodgate driving motor in response to a measurement result of the inflow amount measurement sensor. A wastewater level is adjusted within a predetermined range through operation of the vertical floodgate to maintain a constant flow velocity of wastewater which passes through a sand sedimentation zone, thereby preventing organic material which is contained in the wastewater and is other than sand, from sedimenting.