An electric supercharger, comprising: a compressor impeller; a motor configured to transmit a driving force to the compressor impeller via a rotational shaft; a back-surface side casing facing a back surface of the compressor impeller via a gap (g) and surrounding the rotational shaft; a bearing (10A, 10B) disposed between the back-surface side casing and the rotational shaft so as to rotatably support the rotational shaft; and a mechanical seal positioned between the back surface of the compressor impeller and the bearing (10A, 10B) in an axial direction of the compressor impeller and configured to seal a gap between the rotational shaft and the back-surface side casing, wherein the mechanical seal includes: a stationary ring supported on the back-surface side casing; a rotary ring protruding from the rotational shaft toward an outer side in a radial direction of the compressor impeller and facing the stationary ring so as to be capable of being in contact with the stationary ring in the axial direction of the compressor impeller, the rotary ring being configured to rotate with the rotational shaft; and a biasing member configured to bias one of the rotary ring or the stationary ring toward the other one of the rotary ring or the stationary ring, and wherein a groove is formed on a facing surface (32, 21) which is one of a surface of the rotary ring which faces the stationary ring or a surface of the stationary ring which faces the rotary ring, wherein the groove on the facing surface (32,21) is formed so as to be in communication with a first space on an inner side of the facing surface (32,21) in the radial direction between the stationary ring and the rotational shaft, and so as not to be in communication with a second space on an outer side of the facing surface (32,21) in the radial direction of the gap (g) between the back surface and the back-surface side casing in a state where the stationary ring and the rotary ring are in contact. a compressor impeller; a motor configured to transmit a driving force to the compressor impeller via a rotational shaft; a back-surface side casing facing a back surface of the compressor impeller via a gap (g) and surrounding the rotational shaft; a bearing (10A, 10B) disposed between the back-surface side casing and the rotational shaft so as to rotatably support the rotational shaft; and a mechanical seal positioned between the back surface of the compressor impeller and the bearing (10A, 10B) in an axial direction of the compressor impeller and configured to seal a gap between the rotational shaft and the back-surface side casing, wherein the mechanical seal includes: a stationary ring supported on the back-surface side casing; a rotary ring protruding from the rotational shaft toward an outer side in a radial direction of the compressor impeller and facing the stationary ring so as to be capable of being in contact with the stationary ring in the axial direction of the compressor impeller, the rotary ring being configured to rotate with the rotational shaft; and a biasing member configured to bias one of the rotary ring or the stationary ring toward the other one of the rotary ring or the stationary ring, and a stationary ring supported on the back-surface side casing; a rotary ring protruding from the rotational shaft toward an outer side in a radial direction of the compressor impeller and facing the stationary ring so as to be capable of being in contact with the stationary ring in the axial direction of the compressor impeller, the rotary ring being configured to rotate with the rotational shaft; and a biasing member configured to bias one of the rotary ring or the stationary ring toward the other one of the rotary ring or the stationary ring, and wherein a groove is formed on a facing surface (32, 21) which is one of a surface of the rotary ring which faces the stationary ring or a surface of the stationary ring which faces the rotary ring, wherein the groove on the facing surface (32,21) is formed so as to be in communication with a first space on an inner side of the facing surface (32,21) in the radial direction between the stationary ring and the rotational shaft, and so as not to be in communication with a second space on an outer side of the facing surface (32,21) in the radial direction of the gap (g) between the back surface and the back-surface side casing in a state where the stationary ring and the rotary ring are in contact.