Terahertz measuring device for measuring test objects, in particular of plastics material, having - at least one transmitter/receiver unit (3; 3, 3') having -- a transmitter for emitting terahertz radiation in the terahertz frequency range from 0.01 THz to 50 THz, -- an associated receiver for detecting a radiation which is reflected on the test object in the terahertz frequency range from 0.01 THz to 50 THz, - at least one mirror arrangement (4; 4, 4') having -- a first mirror (7; 7, 7'), --- which is constructed in an x-y plane (E xy; E xy, E xy ') with a first focal point (B 1; B 1, B 1 ') and a second focal point (B 2; B 2, B 2 '), and --- which is curved at least partially in an elliptical manner in order to redirect the radiation (S, R) between the focal points (B 1, B 2; B 1, B 2, B 1 ', B 2 '), -- having a second mirror (8; 8, 8') which is arranged in the region of the first focal point (B 1; B 1, B 1 ') in order to redirect the radiation (S, R) between a z direction which extends transversely relative to the x-y plane (E xy; E xy, E xy ') and the x-y plane (E xy; E xy, E xy '), - a test object holder for arranging the test object in the region of the second focal point (B 2; B 2, B 2 '), and - having a control unit, wherein the control unit is connected to the transmitter and the receiver for controlling the transmitter and evaluating the reflected radiation detected by the receiver and establishing a wall thickness (dw) or layer thickness of the test object. - at least one transmitter/receiver unit (3; 3, 3') having -- a transmitter for emitting terahertz radiation in the terahertz frequency range from 0.01 THz to 50 THz, -- an associated receiver for detecting a radiation which is reflected on the test object in the terahertz frequency range from 0.01 THz to 50 THz, -- a transmitter for emitting terahertz radiation in the terahertz frequency range from 0.01 THz to 50 THz, -- an associated receiver for detecting a radiation which is reflected on the test object in the terahertz frequency range from 0.01 THz to 50 THz, - at least one mirror arrangement (4; 4, 4') having -- a first mirror (7; 7, 7'), --- which is constructed in an x-y plane (E xy; E xy, E xy ') with a first focal point (B 1; B 1, B 1 ') and a second focal point (B 2; B 2, B 2 '), and --- which is curved at least partially in an elliptical manner in order to redirect the radiation (S, R) between the focal points (B 1, B 2; B 1, B 2, B 1 ', B 2 '), -- having a second mirror (8; 8, 8') which is arranged in the region of the first focal point (B 1; B 1, B 1 ') in order to redirect the radiation (S, R) between a z direction which extends transversely relative to the x-y plane (E xy; E xy, E xy ') and the x-y plane (E xy; E xy, E xy '), -- a first mirror (7; 7, 7'), --- which is constructed in an x-y plane (E xy; E xy, E xy ') with a first focal point (B 1; B 1, B 1 ') and a second focal point (B 2; B 2, B 2 '), and --- which is curved at least partially in an elliptical manner in order to redirect the radiation (S, R) between the focal points (B 1, B 2; B 1, B 2, B 1 ', B 2 '), --- which is constructed in an x-y plane (E xy; E xy, E xy ') with a first focal point (B 1; B 1, B 1 ') and a second focal point (B 2; B 2, B 2 '), and --- which is curved at least partially in an elliptical manner in order to redirect the radiation (S, R) between the focal points (B 1, B 2; B 1, B 2, B 1 ', B 2 '), -- having a second mirror (8; 8, 8') which is arranged in the region of the first focal point (B 1; B 1, B 1 ') in order to redirect the radiation (S, R) between a z direction which extends transversely relative to the x-y plane (E xy; E xy, E xy ') and the x-y plane (E xy; E xy, E xy '), - a test object holder for arranging the test object in the region of the second focal point (B 2; B 2, B 2 '), and - having a control unit,