A muscle electrostimulation device comprising: a main body having a contact surface for fitting around an abdomen, wherein the contact surface extends in an X direction encircling the abdomen and in a Y direction perpendicular to the X direction, the main body including: a front part having a front electrode unit comprising a plurality of electrode groups (32b, 32c) spaced apart in the X direction to cause an electric current in the X direction in an abdominal rectus muscle; and a pair of extensions extending from respective sides of the front part in the X direction and each having a respective side electrode unit comprising a plurality of electrodes spaced apart in the X direction to cause an electric current in the X direction to flow in a respective oblique abdominal muscle, wherein: boundary recesses that recede in the Y direction are provided on respective sides of the main body along the Y direction at respective boundaries between the front part and the pair of extensions; and front recesses that recede in the Y direction are provided at positions in side portions of the front part along the Y direction that bisect the front part in the X direction. the contact surface extends in an X direction encircling the abdomen and in a Y direction perpendicular to the X direction, the main body including: a front part having a front electrode unit comprising a plurality of electrode groups (32b, 32c) spaced apart in the X direction to cause an electric current in the X direction in an abdominal rectus muscle; and a pair of extensions extending from respective sides of the front part in the X direction and each having a respective side electrode unit comprising a plurality of electrodes spaced apart in the X direction to cause an electric current in the X direction to flow in a respective oblique abdominal muscle, wherein: boundary recesses that recede in the Y direction are provided on respective sides of the main body along the Y direction at respective boundaries between the front part and the pair of extensions; and front recesses that recede in the Y direction are provided at positions in side portions of the front part along the Y direction that bisect the front part in the X direction. a front part having a front electrode unit comprising a plurality of electrode groups (32b, 32c) spaced apart in the X direction to cause an electric current in the X direction in an abdominal rectus muscle; and a pair of extensions extending from respective sides of the front part in the X direction and each having a respective side electrode unit comprising a plurality of electrodes spaced apart in the X direction to cause an electric current in the X direction to flow in a respective oblique abdominal muscle, wherein: boundary recesses that recede in the Y direction are provided on respective sides of the main body along the Y direction at respective boundaries between the front part and the pair of extensions; and front recesses that recede in the Y direction are provided at positions in side portions of the front part along the Y direction that bisect the front part in the X direction. boundary recesses that recede in the Y direction are provided on respective sides of the main body along the Y direction at respective boundaries between the front part and the pair of extensions; and front recesses that recede in the Y direction are provided at positions in side portions of the front part along the Y direction that bisect the front part in the X direction.