To provide a structure of installing an armrest of a chair which enables a user to install an armrest easily and securely to a chair without a flat surface on a part to install the armrest.

A lower part of a cylindrical, armrest pole 9, which has an armrest 10 at its upper part and a female screw 11 at its lower part, is fit into an upper end of a cylindrical leg 2 with a hole 2b for penetrating a screw on its upper periphery in a manner that there is a space between the armrest pole 9 and the cylindrical leg 2 for movements. A lower part support member 13 with an extended collar 13b the periphery of which makes contact with the inner circumference of the leg 2 is placed at a lower end of the armrest pole 9. An upper part support member part 12 which is fit around the armrest pole 9 at a higher position than that the female screw 11 is inserted into an upper end of the leg 2. A screw 6 is penetrated through the hole 2b for penetrating a screw and is screwed with the female screw 11 to fasten between the upper part support member 12 and the lower part support member 13.