A powerless automatic flushing toilet seat for a water tank, the toilet seat comprising: a body sensing unit (100, 100') pressing seat springs and guiding a toilet seat that moves down when a user sits on the toilet seat, and guiding the toilet seat that moves up using a restoring force of the seat springs and sensing a human body when the user stands up from the toilet seat; an operation unit including a loading member having a loading force for pulling a flushing wire connectable to a siphon cover in response to downward movement of the body sensing unit, and removing excreta with rotation of the siphon cover by removing a loading force of the loading member and pulling the flushing wire through an unloading member operating in response to upward movement of the body sensing unit; and an unloading unit keeping a loading force in the flushing wire in response to downward movement of the body sensing unit and removing the kept loading force in response to upward movement of the body sensing unit; characterized in that the powerless automatic flushing toilet seat has air exhaust and intake functions only through human body weight, and that the toilet seat further comprises an adjusting unit adjusting operation time of the operation unit by discharging internal air in response to downward movement of the body sensing unit and by adjusting the amount of air flowing inside in response to upward movement of the body sensing unit at a compressed level of an air adjusting filter by a bolt, wherein the adjusting unit includes: an adjusting body having an air exhaust hole and being fixed to the toilet seat; a cover member opening and closing the air exhaust hole; an anti-separation member preventing separation of the cover member; and a vertical guide member discharging air in the adjusting body to the air exhaust hole by vertically moving in the adjusting body and being fixed to the body sensing unit to suck air outside the adjusting body. a body sensing unit (100, 100') pressing seat springs and guiding a toilet seat that moves down when a user sits on the toilet seat, and guiding the toilet seat that moves up using a restoring force of the seat springs and sensing a human body when the user stands up from the toilet seat; an operation unit including a loading member having a loading force for pulling a flushing wire connectable to a siphon cover in response to downward movement of the body sensing unit, and removing excreta with rotation of the siphon cover by removing a loading force of the loading member and pulling the flushing wire through an unloading member operating in response to upward movement of the body sensing unit; and an unloading unit keeping a loading force in the flushing wire in response to downward movement of the body sensing unit and removing the kept loading force in response to upward movement of the body sensing unit; characterized in that the powerless automatic flushing toilet seat has air exhaust and intake functions only through human body weight, and that the toilet seat further comprises an adjusting unit adjusting operation time of the operation unit by discharging internal air in response to downward movement of the body sensing unit and by adjusting the amount of air flowing inside in response to upward movement of the body sensing unit at a compressed level of an air adjusting filter by a bolt, wherein the adjusting unit includes: an adjusting body having an air exhaust hole and being fixed to the toilet seat; a cover member opening and closing the air exhaust hole; an anti-separation member preventing separation of the cover member; and a vertical guide member discharging air in the adjusting body to the air exhaust hole by vertically moving in the adjusting body and being fixed to the body sensing unit to suck air outside the adjusting body. an adjusting body having an air exhaust hole and being fixed to the toilet seat; a cover member opening and closing the air exhaust hole; an anti-separation member preventing separation of the cover member; and a vertical guide member discharging air in the adjusting body to the air exhaust hole by vertically moving in the adjusting body and being fixed to the body sensing unit to suck air outside the adjusting body.