To provide a waterproof structure of a member tightening part by a screw capable of allowing repetitive use without damaging a member and preventing a required space from becoming large.

In this waterproof structure of the tightening part for tightening a first member 1 having a bottomed screw hole 1a and a second member 2 having a screw insertion hole 2a by a screw 4 inserted into the screw insertion hole 2a and screw-fitted into the screw hole 1a, the screw 4 is a flat countersunk head screw, an annular channel 3b being coaxial with a recessed conical face 3a is provided on an outer face of a spacer 3 having an insertion hole forming a recessed conical face 3a fitted into the screw insertion hole 2a and coming into face-contact with a head part conical face of the flat countersunk head screw 4, an O-ring 5 is fitted into the annular channel 3b, a spacer 3 is provided between the first member and the second member to tighten the flat countersunk head screw 4, and the O-ring 5 is brought into pressure-contact with an inner face of the annular channel 3b and a face of the screw insertion hole of the second member 2.