This paper introduces an approach to design adaptive disturbance observer on mechatronic systems. Disturbance observer has been presented by many researchers and engineers. Similarly, adaptive control methodology including adaptive observer has been studied for long periods. Then, the observer which posssess the both functions of disturbance estimation and parameter estimation seems to be useful to practical mechatronic systems. This paper presents a practical design of K-type adaptive observer which estimates not only normal state variables but also unknown disturbance.