This paper deals with numerical analysis of sloshing problems for the middle-sized double hull tanker. A numerical simulation method for non-linear liquid sloshing in the cargo oil tank is proposed by making up a new sloshing cord based upon the SOLA-SURF scheme and a simple two-dimensional sloshing cord. The new sloshing cord has many functions for analyzing sloshing liquid violence motions in the octagon-shaped tank fitted with centre girder, deck centre girder, deck side girder, horizontal girder etc. A series of numerical simulations are executed for cargo oil tanks of a middle-sized double hull tanker and of a single hull tanker sailing at forward speed in regular waves, and numerical calculations are executed for four model tanks in order to compare with experimental results too. By numerical simulations, it is confirmed that the sloshing cord is useful for analyzing non-linear sloshing and for estimating sloshing pressure in liquid cargo tanks of a double hull tanker.