PURPOSE: To prevent a columnar buoy from inclining, by providing such an arrangement that a resisting member is projected downward from the lower section of the buoy so that a moment generated about the mooring wire end section 14 in the lower section of the buoy due to the tide is cancelled out by a moment produced by the resisting member.

CONSTITUTION: A tide resisting member 15 is attached to a buoy 11 so that it is projected downward from a mooring wire connecting end section 14. Thereby, the tide resisting member 15, when the buoy 11 and the resisting member 15 are subjected to the affection of the tide A, applies a moment M2 to the buoy 11 which has a magnitude equal to that of a moment M1 which is exerted upon the buoy 11 horizontally about the mooring wire connecting end section 14, but in the direction opposite to the moment M1. Further, a mooring wire 12 is connected to the lower end of the buoy 11, and with this mooring wire 12 the attitude of the buoy 11 which is vertical long and which carries an observing apparatus that is moored at a predetermined depth below the sea surface, is efficiently compensated to its vertical position.