A DNP-NMR probe comprising: an inner container in which a sample to which a radical is added is installed; an outer container that stores the inner container therein and keeps a space between the outer container and the inner container in a vacuum state; an outer container waveguide that is a microwave waveguide provided on the outer container and has a vacuum window at an end on the inner container side; and an inner container waveguide that is a microwave waveguide provided on the inner container and has a vacuum window facing the vacuum window of the outer container waveguide through vacuum, the inner container waveguide guiding microwaves transmitted from the outer container waveguide to the sample, a microwave transmitter arranged to transmit microwaves whose wavelength in a vacuum is λ 0, characterized in that the window thickness of the vacuum window of the outer container and the vacuum window of the inner container is optimized for the transmittance of the microwaves transmitted by the microwave transmitter, wherein the window thickness of the vacuum window of the outer container Dout and the window thickness of the vacuum window of the inner container Din are respectively set within the ranges of Dout 0 − Δ Dout / 8 ≤ Dout ≤ Dout 0 + Δ Dout / 8 Din 0 − Δ Din / 8 ≤ Din ≤ Din 0 + Δ Din / 8 for the specific microwaves used, where Dout 0 and Din 0 are window thicknesses at which the microwave transmittance exhibits a local maximum when the respective window thicknesses Dout and Din are changed, and respectively satisfy relationships of Dout 0 = i / 2 + 1 / 4 λ ′ and Din 0 = j / 2 + 1 / 4 λ ′ where each of i and j is an integer that is 0 or greater, and ΔDout and ΔDin are distances that provide a cycle in which the microwave transmittance changes when the respective window thicknesses Dout and Din are changed, and respectively satisfy relationships of Δ Dout = λ ′ / 2 and Δ Din = λ ′ / 2 where λ' is the wavelength of the microwaves passing through the vacuum windows and satisfies a relationship of λ ′ =λ 0 / n where n is the index of refraction of the vacuum windows. an inner container in which a sample to which a radical is added is installed; an outer container that stores the inner container therein and keeps a space between the outer container and the inner container in a vacuum state; an outer container waveguide that is a microwave waveguide provided on the outer container and has a vacuum window at an end on the inner container side; and an inner container waveguide that is a microwave waveguide provided on the inner container and has a vacuum window facing the vacuum window of the outer container waveguide through vacuum, the inner container waveguide guiding microwaves transmitted from the outer container waveguide to the sample, a microwave transmitter arranged to transmit microwaves whose wavelength in a vacuum is λ 0, characterized in that the window thickness of the vacuum window of the outer container and the vacuum window of the inner container is optimized for the transmittance of the microwaves transmitted by the microwave transmitter, wherein the window thickness of the vacuum window of the outer container Dout and the window thickness of the vacuum window of the inner container Din are respectively set within the ranges of Dout 0 − Δ Dout / 8 ≤ Dout ≤ Dout 0 + Δ Dout / 8 Din 0 − Δ Din / 8 ≤ Din ≤ Din 0 + Δ Din / 8 for the specific microwaves used, where Dout 0 and Din 0 are window thicknesses at which the microwave transmittance exhibits a local maximum when the respective window thicknesses Dout and Din are changed, and respectively satisfy relationships of Dout 0 = i / 2 + 1 / 4 λ ′ and Din 0 = j / 2 + 1 / 4 λ ′ where each of i and j is an integer that is 0 or greater, and ΔDout and ΔDin are distances that provide a cycle in which the microwave transmittance changes when the respective window thicknesses Dout and Din are changed, and respectively satisfy relationships of Δ Dout = λ ′ / 2 and Δ Din = λ ′ / 2 where λ' is the wavelength of the microwaves passing through the vacuum windows and satisfies a relationship of λ ′ =λ 0 / n where n is the index of refraction of the vacuum windows.