A picture (P) is coded in inter-mode according to scene changes. Direct-mode motion vectors are calculated and coded according to the type of reference picture pointed to by a motion vector of a co-located block in the specified picture. The type of reference picture is determined using a reference picture index calculated previously at a co-located block. The index is stored in a system buffer. When a motion vector points to a long-term reference picture, the forward motion vector is for the co-located block and the backwards motion vector is zero. When a motion vector points to a short term reference picture, motion vectors are determined by scaling the motion vector of the co-located block in the specified picture by using the time difference between the pictures. Motion vectors are stored in a system buffer. Independent claims are also included for the following: (1) a method of coding a moving picture in inter-mode; and (2) a method of coding a moving picture sequence in a moving picture coding system.