Attenuation of microwaves is reduced or prevented when vacuum windows are provided in microwave waveguides of a DNP-NMR probe. A DNP-NMR probe has an inner container and an outer container. The inner container has therein a sample tube containing a sample to which radicals are added. The outer container keeps a space between the outer container and the inner container in the outer container in a vacuum state. The outer container has an outer container waveguide that has a vacuum window at an inner end portion and guides microwaves. The inner container has a vacuum window facing the vacuum window of the outer container waveguide through vacuum, and guides microwaves transmitted from the outer container waveguide to the sample. The window-to-window distance between the vacuum window of the outer container and the vacuum window of the inner container is adjusted by means of adjustment bolts.