A waterproof structure for a tightening section where a first member (1) having a screw hole (1a) with a bottom and a second member (2) having a screw insertion hole (2a) are fastened with a screw (4) that is inserted into the screw insertion hole (2a) and screwed to the screw hole (1a). The screw (4) is a flat head screw, and an annular groove (3b) is formed in the outer surface of a spacer (3) having an insertion hole where recessed circular conical surface (3a) is formed. The spacer (3) is fitted in the screw insertion hole (2a) to be in surface contact with a circular conical surface of a head of the flat head screw. The annular groove (3b) is coaxial with the recessed circular conical surface (3a). An O-ring (5) is installed in the annular groove (3b), and the flat head screw (4) is tightened with the spacer (3) placed between first member and the second member, causing the O-ring (5) to be pressed between an inner surface of the annular groove (3b) and a surface of the screw insertion hole of the second member (2). The waterproof structure can be repeatedly used without damaging the members and does not enlarge a necessary space.