We fabricated a 300-m-long multifilamentary MgB 2 wire as a powder in tube wire by using an in situ method. The J e values were approximately 250 A/mm 2 from 2 to 3 T at 20 K and 3 to 4 T at 15 K. These J e values were interpolated by J c values on monofilament wire because I c measurements over 100 A were difficult due to joule heat on solders. By using this 300 m wire, a coil was designed to be driven in the maximum magnetic flux density in MgB 2 filaments (B max ) over 2 T. The coil was made by using a wind-and-react method and cooled by using conduction cooling. The coil was successfully driven at 24 K and 27 K as temperatures of coil winding, and calculated B max was 2.3 T in 24 K operation. The coil I c values measured at 24 K and 27 K agreed well with the estimated values based on I c values on short-length wire. This means that the 300-m-long multifilamentary wire has sufficient longitudinal homogeneity for designing MgB 2 coils and can be used at a maximum magnetic flux density over 2 T.