Sealing unit and projector

A sealing unit, having:

• a case which comprises an opening portion on an inner circumference of which an attachment surface is formed;
• a seal member which comprises an annular flat plate portion whose side edge surfaces constitute an inner and outer circumferential surfaces and an annular rib portion which is provided on one flat plate surface of the annular flat plate portion so as to rise therefrom and which is made of an elastic material;
• an attachment member which comprises an annular attachment surface on an outer circumference thereof;
• a fixing member;
• and
• a fastening member, wherein
• an outer circumferential surface of the annular rib portion of the seal member is brought into abutment with the attachment surface of the opening portion, wherein
• an inner circumferential surface of the annular rib portion of the seal member is brought into abutment with the attachment surface of the attachment member, wherein
• the fixing member is brought into abutment with the other flat plate surface of the seal member and is fixed to the attachment member, and wherein
• the fastening member is brought into abutment with the other flat surface of the seal member and is fixed to the case.