They first checked the water pressure for 15 mins but it didn't drop, so they ruled out the pipe problem. They suggested me not to use the toilet for a week, if no more leaking, then it has to be the floor problem.
After a week, they found it strange that the bathroom downstairs still leaks. The only possibility now is thewater actually came from the air-con pipe. They then suggested me not to switch on the air-con and cover any gap between tiles with water proof cement.
When I went to hardware shop, they told me that water proof cement is sold in 25kg, and only used under the tiles. This means that the floor must be hacked. As I try not to do any hacking, I bought a liquid sealant and squeeze the whole 500ml bottle into all the holes i could find between the tiles. Not feeling that this will work, I went bought another water proof sealer. First covered the holes with white cement, waited for a day to dry, washed the floor, waited for another day to dry and applied the sealer 3 times.
Ta...da...Look at the tiles, it's still wet and shining.
I have sacrificed many days trying to fix this thing, just hope it will work.
Will see...