A bone-conduction hearing aid device is provided. The bone-conduction hearing aid device is suitable for being attached to a body surface and includes a substrate, an input transducer, an amplifier, and a bone-conduction speaker. The substrate is composed of a plurality of stacking layers stacked on top of one another. A material of the substrate includes cellulose nanofiber. The substrate is formed by 3D printing technique so that a contact surface of the substrate is tightly attached with the body surface. The input transducer is disposed on the substrate and configured to receive a sound signal and convert the sound signal into an electric signal. The amplifier is disposed on the substrate and coupled to the input transducer to amplify the electric signal into an amplified electric signal. The bone-conduction speaker is disposed on the substrate and coupled to the amplifier to convert the amplified electric signal into a vibration signal.