It is known that there is a possibility that severe sloshing can occur in partially filled cargo tanks of membrane LNG carriers and that it may cause damage to the tank structures. Recently, new modes of LNG cargo transportation have arisen, such as the use of LNG carriers in the shuttle transportation of natural gas from floating LNG, the adoption of LNG as a fuel for ships, etc. In these new applications, partial filling in the cargo tank or in the fuel tank is inevitable. In this study, in order to investigate the sloshing characteristic in partially filled membrane tanks, sloshing model experiments were carried out. In the experiments, a strong rotational motion of the free surface in the tank, i.e. swirling, was observed. Based on the experimental and numerical studies carried out in this study, it was found that swirling occurs when the tank length to tank breadth ratio ( L_t / B_t ) is close to 1. In addition, even in irregular excitation, it was confirmed that there is a possibility that swirling occurs with the same condition. There are some actual ships that have tanks which conform with the above mentioned condition, so swirling may occur if such ships are operated in partially filled condition. Furthermore, the relationship between the sloshing force and the applied excitation was investigated from the results of irregular excitation experiments and numerical calculations. Based on the derived formula, a distribution map of sloshing load in the North Pacific Ocean was obtained.