When your garage door opens and closes there are two switches that it trips (one for when it opens and one for when it closes). These will normally be by the belt drive casing or near the railing at the sides. These can be adjusted forwards or backwards to control how far the door opens and how far it closes. What is probably causing your problem is that the switch that gets tripped when your garage door closes is too far forward allowing it to hit the ground before it trips the switch to stop the garage door. Garage doors have a pressure sensitive mechanism on them for safety reasons. If the door hits an object when it is closing it will stop and raise back up. This is so it won't crush a kid or a person if they happed to be under the garage door when it closes. Since your switch to stop the garage door is too far forward or possibly broken your garage door sensors think the door has hit an obstruction when it hits the ground and therefore it raises. If adjusting the trigger switch doesn't work, you might try adjusting the pressure it takes to send the garage door back up. It could possibly be too sensitive. That adjustment should be located on the garage door motor. Only adjust that AFTER you have already tried moving the trigger to stop the garage door when it closes.
위에서 밀레니엄 님이 설명하신 대로 콘트롤 박스 안에 있는 닫힘 스위치가 차고문이 바닥에 닿을 때까지도 접촉이 안되는 것 같네요. 그러니까 Down (닫힘) 쪽에 있는 나사를 조정해야 하는데 풀어야 하는지 감아야 하는지는 구조를 정확히 몰라서 설명하기가 곤란하군요. Down 쪽의 나사를 풀거나 감으면서 조절을 해보셔야 하겠네요.