We Claim:
1. A video image decoding device comprising: a variable length decoder that performs a variable-length decoding process on coded data multiplexed into a bitstream to obtain index information which specifies a motion vector for a coding block and control information for varying the number of motion vector candidates for each slice; a motion compensator that performs a motion compensation prediction process on said coding block to generate a prediction image using a motion vector selected from one or more selectable motion vector candidates, said motion compensator selecting said motion vector from said motion vector candidates according to said index information; wherein said motion compensator prepares a list of said motion vector candidates including one or more spatial motion vectors which is obtained from a decoded block located around said coding block and/or a temporal motion vector which is obtained from a different picture which can be referred to by said coding block, said motion vector candidates being listed in accordance with a predetermined order, and said index information specifying a position in which one of said motion vector candidates exists within said list.
2. A video image decoding method comprising: performing a variable-length decoding process on coded data multiplexed into a bitstream to obtain index information which specifies a motion vector for a coding block and control information for varying the number of motion vector candidates for each slice; performing a motion compensation prediction process on said coding block to generate a prediction image using a motion vector selected from one or more selectable motion vector candidates, said motion vector being selected from said motion vector candidates according to said index information; wherein said motion vector candidates include one or more spatial motion vectors which is obtained from a decoded block located around said coding block and/or a temporal motion vector which is obtained from a decoded picture which can be referred to by said coding block, said motion vector candidates being listed in accordance with a predetermined order, and said index information specifying a position in which one of said motion vector candidates exists within said list.

3. A video image encoding device comprising: a motion compensator that performs a motion compensation prediction process on a coding block to generate a prediction image using a motion vector selected from one or more selectable motion vector candidates, said motion compensator selecting said motion vector from said motion vector candidates; a variable encoder that performs a variable-coding process to generate index information which specifies selected one of said motion vector candidates and control information for varying the number of said motion vector candidates for each slice; wherein said motion compensator prepares a list of said motion vector candidates including one or more spatial motion vectors which is obtained from a coded block located around said coding block and/or a temporal motion vector which is obtained from a different picture which can be referred to by said coding block, said motion vector candidates being listed in accordance with a predetermined order, and said index information specifying a position in which one of said motion vector candidates exists within said list.
4. A video image encoding method comprising: performing a motion compensation prediction process on a coding block to generate a prediction image using a motion vector selected from one or more selectable motion vector candidates, said motion compensator selecting said motion vector from said motion vector candidates; performing a variable-coding process to generate index information which specifies selected one of said motion vector candidates and control information for varying the number of said motion vector candidates for each slice; wherein said motion compensator prepares a list of said motion vector candidates including one or more spatial motion vectors which is obtained from a coded block located around said coding block and/or a temporal motion vector which is obtained from a different picture which can be referred to by said coding block, said motion vector candidates being listed in accordance with a predetermined order, and said index information specifies a position in which one of said motion vector candidates exists within said list.