A mouth guard which is excellent in impact absorbability and durability, is thin and has good fittability, and is able to easily chew thereupon, and does not give vomiting feel; and a sheet for mouth guard which is suitable for producing the mouth guard. The mouth guard or the sheet for mouth guard comprises at least one block copolymer selected from the group consisting of a block copolymer (a-1) comprising polymer blocks A made of a vinyl aromatic compound, and polyisoprene blocks B having a content of 1,2-bonds and 3,4-bonds of 40% by mol or more; a block copolymer (a-2) comprising polymer blocks A, and polymer blocks C which contain an isoprene unit and a butadiene unit in a ratio of from 5/95 to 95/5 (% by weight) and have a content of 1,2-bonds and 3,4-bonds of 40% by mol or more; and a block copolymer (a-3) comprising polymer blocks A, and polybutadiene blocks D having a content of 1,2-bonds of 60% by mol or more, wherein a content of vinyl aromatic compound units in each of the copolymers is from 10 to 40% by weight, and a hydrogenated product thereof (b-1) to (b-3).