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 movable member 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 trunable between an unlocking position where said movable member is pivotable in relation to said fixed member for adjustment of the 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 the securement of said backrest relative to said fixed member when locked by said fixing member is reinforced against release when a person sitting on said seat leans against said backrest.