PURPOSE: To enable insertion into fuel aggregate without decomposing the fuel aggregate, by installing spacer combs whose number is one-half of the number of sides of a polygon, and making the spacer combs vertically overlap with each other.

CONSTITUTION: This spacer has a lower side comb 6 and an upper side comb 8. The comb 6 has unit terminal band plates 10 whose length correspond to the width of a fuel aggregate. This is defined by the width of a supporting plate 2. A plurality of grating belt plates 12 stretch rectangularly to the terminal band plate 10. The grating belt plate 12 is doubly formed of spring material, and provided with a slant channel 14 which guides cooling water so as to generate a swirl around a fuel rod 4. These form together a perfect grating spacer maintaining the position of the fuel rods. The spacer can be inserted into the fuel aggregate, even in water, by using a slender handling jig.