1. A moving picture coding method of coding a current block included in a current picture using a reference motion vector of a reference block included in a reference picture different from the current picture, the reference block being co-located, in the reference picture, with the current block in the current picture, the moving picture coding method comprising: determining a value of a predetermined flag which indicates whether or not reference block information is to be read from a memory, the reference block information including, among reference motion vectors of the reference block in a first prediction direction and a second prediction direction, the reference motion vector in the first prediction direction; reading, from the memory, the reference block information including the reference motion vector according to the value of the predetermined flag; coding a motion vector of the current block using the read reference block information; writing, to the memory, the reference block information including the motion vector of the current block according to the value of the predetermined flag; and adding the predetermined flag to a bitstream.