An oral appliance (1000, 4000), comprising: a pair of upper and lower jaw oral appliances (1110, 1120, 1210, 1220, 4210, 4220, 4310, 4320) facing each other; a pair of positioning members (1140, 1240, 4240, 4340) each being adjustable in position in a front-rear direction, the pair of positioning members (1140, 1240, 4240, 4340) being provided in one oral appliance (1120, 1220, 4220, 4320) of the upper and lower jaw oral appliances (1110, 1120, 1210, 1220, 4210, 4220, 4310, 4320); and a pair of stoppers (1130, 1230, 4230, 4330) provided in another oral appliance (1110, 1210, 4210, 4310) of the upper and lower jaw oral appliances (1110, 1120, 1210, 1220, 4210, 4220, 4310, 4320), the pair of stoppers (1130, 1230, 4230, 4330) abutting against the respective positioning members (1140, 1240, 4240, 4340) and thereby restricting rearward displacement of the lower jaw oral appliance (1120, 1220, 4220, 4320), and each of the stoppers (1130, 1230, 4230, 4330) including an extension portion (1134, 1234, 4234, 4334) that abuts against the oral appliance (1120, 1220, 4220, 4320) thereby restricting displacement in a right-left direction of the upper jaw oral appliance (1110, 1210, 4210, 4310) and lower jaw oral appliance (1120, 1220, 4220, 4320), the extension portion (1134, 1234, 4234, 4334) having a height that is equal to or less than half a height of the stoppers (1130, 1230, 4230, 4330), and characterized in that each of the stoppers (1130, 1230, 4230, 4330) include an inclined surface (1136, 1236, 4236, 4336) which abuts against relevant positioning members (1140, 1240, 4240, 4340), and the inclined surface (1136, 1236, 4236, 4336) is further inclined in the extension portion (1134, 1234, 4234, 4334). a pair of upper and lower jaw oral appliances (1110, 1120, 1210, 1220, 4210, 4220, 4310, 4320) facing each other; a pair of positioning members (1140, 1240, 4240, 4340) each being adjustable in position in a front-rear direction, the pair of positioning members (1140, 1240, 4240, 4340) being provided in one oral appliance (1120, 1220, 4220, 4320) of the upper and lower jaw oral appliances (1110, 1120, 1210, 1220, 4210, 4220, 4310, 4320); and a pair of stoppers (1130, 1230, 4230, 4330) provided in another oral appliance (1110, 1210, 4210, 4310) of the upper and lower jaw oral appliances (1110, 1120, 1210, 1220, 4210, 4220, 4310, 4320), the pair of stoppers (1130, 1230, 4230, 4330) abutting against the respective positioning members (1140, 1240, 4240, 4340) and thereby restricting rearward displacement of the lower jaw oral appliance (1120, 1220, 4220, 4320), and each of the stoppers (1130, 1230, 4230, 4330) including an extension portion (1134, 1234, 4234, 4334) that abuts against the oral appliance (1120, 1220, 4220, 4320) thereby restricting displacement in a right-left direction of the upper jaw oral appliance (1110, 1210, 4210, 4310) and lower jaw oral appliance (1120, 1220, 4220, 4320), the extension portion (1134, 1234, 4234, 4334) having a height that is equal to or less than half a height of the stoppers (1130, 1230, 4230, 4330), and characterized in that each of the stoppers (1130, 1230, 4230, 4330) include an inclined surface (1136, 1236, 4236, 4336) which abuts against relevant positioning members (1140, 1240, 4240, 4340), and the inclined surface (1136, 1236, 4236, 4336) is further inclined in the extension portion (1134, 1234, 4234, 4334).