A memory device installed in a backrest of a chair is disclosed. The memory device includes a plurality of support shafts, a plurality of housings receiving the plurality of support shafts, a support block inserted in the respective housings and having a plurality of fixing grooves formed on both sides thereof, a spring for resilient supporting the support block, a setting block having one end rotatably receiving the support shaft, in a state where the setting block is inserted in both sides of the housing, and the other end with a fixing boss inserted in the fixing groove, a setting spring for resiliently supporting the setting block, a rotary lever connected to the setting block via a wire, a support plate connected to the rotary lever by a hinge, and having a setting groove and a release groove formed on an surface thereof, an operation lever resiliently installed on the rotary lever by a support spring, and a setting pin integrally formed with the operation lever and selectively inserted in the setting groove or the release groove. The memory device further includes a housing having a receiving hole which is stacked on an upper portion of the support shaft, with a portion of a lower part of the backrest of the chair being stacked with the support blocks. The housing is installed at a portion corresponding to cervical vertebra, and has a plurality of pocket grooves, a support pocket is inserted in the respective pocket grooves, and a guide shaft protrudes from the respective pocket grooves.