Influence of inside fluid mass which exists inside the riser pipe on tension variation of the deep sea drilling riser in disconnected mode is investigated. A simple calculation method of the tension variation is proposed taking the inside mass motion into account based on the lumped mass method. This method is verified through the comparison with the tension variation and the riser bottom displacement measured in a basin test using a riser model. The influence of the inside mass on the tension variation is numerically investigated for deep sea drilling risers with 3 different length using the calculation method. For the deep sea riser over 2, 000 m length, the mass corresponding to about 50% of the total inside mass acts on the riser top as the additional tension variation.