In this study, we examined non‐shrinkage firing in the planar direction using low‐temperature co‐fired alumina (LTCA) containing a small quantity of Cu–Ti–Nb–Ag–O additive. A good restraint effect was obtained using AL‐43‐L alumina as a constraint layer whose particle diameter was several times larger (1 to 2 μm) than that of alumina in the LTCA material TM‐5D (0.2 μm). The x‐y shrinkage ratio of the non‐shrinkage fired sample using AL‐43‐L as the constraint layer was within 2 %. However, the density of the non‐shrinkage fired sample was much lower than that of the normal shrinkage fired sample. By producing a powder using a freeze‐dry method, we obtained a well‐sintered alumina with a relative density of approximately 95 % at 950°C, which is lower than the melting point of silver. That is, we achieved both suppression of the shrinkage rate in the x‐y direction and ensured sinterability below the melting point of silver. The thermal conductivity of the sample was 16.2 W/mK, which is extremely high for low‐temperature co‐fired ceramics materials.