Disclosed is a drilling system which has a blow-out preventer (BOP) installed in a sea bottom. The drilling system comprises: a riser which has a side surface opened and closed; an opening and closing door which is installed on the side surface of the riser to open and close the riser; a first mud cutoff device which is horizontally installed inside the riser and cuts off mud flowing downward; and a second mud cutoff device which is horizontally installed inside the BOP and cuts off mud flowing upward. Therefore, the present invention allows equipment to enter the inside of the riser and performs work inside the riser.