Adjustment device of a chair backrest

An adjustment device of a chair backrest, comprising

• a fixed member fixedly fitted to a bottom of a seat of a chair;
• a movable member pivoted to said fixed member, said chair backrest being connected to a rear portion of said movable member from a supporting rod thereof;
• an elastic member biasing said rear portion of said backrest upwards;
• a fixing member connected to an outer end of a bolt passing through both a lateral curved slot of said fixed member and a hole of said movable member from a connecting portion thereof so as to be turnable between an unlocking position where said movable member is pivotable in relation to said fixed member for adjustment of orientation of said backrest in relation to said seat and a locking position where said movable member is fixed to said fixed member;
• and characterized by
• a curved bump formed on said fixed member;
• said curved bump enclosing said curved slot and having a tapering lateral side with an upper portion at least slightly bigger than a lower portion thereof, whereby said backrest is further securely fixed in position when said fixing member is turned to said locking position after adjustment of orientation by means of both said connecting portion of said fixing member and said bolt further moving up relative to said curved bump when a person is sitting on said seat to press said seat down.