Disclosed is an apparatus for adjusting a neck angle of a guitar. The apparatus comprises a support block having a receiving groove into which the neck is inserted. The support block is installed inside a lateral plate. A reinforcing plate is installed at bottom of the receiving groove of the support block. The reinforcing plate includes a central plate, a fixed plate fixed to a front face of the support block, and a guide plate guiding rotating of the neck. An adjustment device includes an adjustment bolt inserted into an adjustment hole and a thread portion formed in the adjustment hole of the central plate of the reinforcing plate so as to be thread-fastened with the adjustment bolt. A fixing device includes a fixing bolt inserted into a fixing hole and a fixing nut fixed inside the neck body and thread-fastened with the fixing bolt.