PROBLEM TO BE SOLVED: To provide an element which allows a terahertz wave to be radiated with high efficiency from a high-refractive-index crystal such as a LN single crystal to an outside.SOLUTION: An electromagnetic wave radiation element 1 comprises: a body 7 made of nonlinear optical crystal; and a sub-wavelength lattice structure 5 formed on the body 7. The sub-wavelength lattice structure 5 includes: protrusions 2 arranged regularly on the body 7 in a first direction X and a second direction Y; and a first and a second groove 3X and 3Y provided between the adjacent protrusions 2 and extending toward X and Y. Each of the protrusions 2 has a pair of first surfaces 2c and 2d opposing in the X-direction, and a pair of second surfaces 2a and 2b opposing in the Y-direction. A width of the first surfaces becomes smaller in the direction from the body 7 to a top edge 2e of the protrusion, and a width of the second surfaces becomes smaller in the direction from the body 7 to the top edge 2e of the protrusion. The first groove has a curved surface when viewed in cross section in a width direction, and the second groove has a curved surface when viewed in cross section in a width direction.