This work aims at a test-time fine-tune scheme to further improve the performance of an already-trained Denoising AutoEncoder (DAE) in the context of semi-supervised audio source separation. Although the state-of-the-art deep learning-based DAEs show sensible denoising performance when the nature of artifacts is known in advance, the scalability of an already-trained network to an unseen signal with an unknown characteristic of deformation is not well studied. To handle this problem, we propose an adaptive fine-tuning scheme where we define a test-time target variables so that a DAE can learn from the newly available sources and the mixing environments in the test mixtures. In the proposed network topology, we stack an AutoEncoder (AE) trained from clean source spectra of interest on top of a DAE trained from a variety of available mixture spectra. Hence, the bottom DAE outputs are used as the input to the top AE, which is to check the purity of the once denoised DAE output. Then, the top AE error is used to fine-tune the bottom DAE during the test phase. Experimental results on audio source separation tasks demonstrate that the proposed fine-tuning technique can further improve the sound quality of a DAE during the test procedure.