A tunnel boring machine for collecting earth excavated upon rotation of a cutter disk includes a non-pressurized chamber behind the cutter disk for collecting earth excavated by the cutter disk and discharging the earth with a carrying fluid. The machine includes a first open tank arranged in the non-pressurized chamber which serves as a vessel for containing the carrying fluid as well as a hopper for collecting excavated earth. Carrying fluid supply apparatus is provided for supplying the carrying fluid to the first open tank and suction/discharge apparatus is provided for sucking and discharging the carrying fluid supply to the first open tank rearwardly together with the collected earth. Water level control apparatus provided for monitoring a water level of a carrying fluid in the first open tank and keeping the water level between a minimum water level and a maximum water level.