To surely and smoothly couple a sleeve fixed to an upper nozzle with a control rod guide tube.

A guide pin unit used for guiding and coupling the sleeve 4 fixed to the upper nozzle 1 to the control rod guide tube 8 is provided with a guide pin 20 whose outer diameter is made compressible at a tip end part 22 and a stopper part 25 by separating with slits 28 and 28. Along the axis line of the tip end part 22 and the stopper part 25 of the guide pin 20, a through hole 21 is formed and a center pin 30 capable of coupling with the penetration hole 21 is provided. An elastic mechanism 50 energizing the tip end side of the guide pin 20 is provided.