A method for manufacturing a diamond substrate, the method comprising: forming a repeated photoresist pattern on a lower substrate; depositing an air gap forming film on the lower substrate and on the repeated photo resist pattern; removing the repeated photoresist pattern by heat treatment and forming an air gap between the lower substrate and the air gap forming film; forming a buffer layer on the air gap forming film; forming a diamond thick film (50, 60) on the buffer layer; and performing cooling so that the air gap forming film on the air gap, the buffer layer, and the diamond thick film are together self-separated from the lower substrate, leaving the air gap forming film, where the air gap is not formed, on the lower substrate; wherein: (i) when the air gap forming film is made of SiC, Si, or a group 3-5 compound semiconductor, the forming of the buffer layer includes sequentially stacking an oxide layer made of a metal oxide containing α-Al 2 O 3 or γ-Al 2 O 3, MgO, SrTiO 3, or YSZ, and a metal layer made of a platinum group metal containing iridium (Ir), platinum (Pt), or rhodium (Rh) or a material containing Ni; or, (ii) when the air gap forming film is made of SiC, Si, or a group 3-5 compound semiconductor, the forming of the buffer layer includes forming a layer containing SiC; or, (iii) when the air gap forming film is made of an Al 2 O 3, MgO, YSZ, iridium, quartz, platinum, or SrTiO 3, the forming of the buffer layer includes forming a metal layer made of a platinum group metal containing iridium (Ir), platinum (Pt), or rhodium (Rh) or a material containing Ni. forming a repeated photoresist pattern on a lower substrate; depositing an air gap forming film on the lower substrate and on the repeated photo resist pattern; removing the repeated photoresist pattern by heat treatment and forming an air gap between the lower substrate and the air gap forming film; forming a buffer layer on the air gap forming film; forming a diamond thick film (50, 60) on the buffer layer; and performing cooling so that the air gap forming film on the air gap, the buffer layer, and the diamond thick film are together self-separated from the lower substrate, leaving the air gap forming film, where the air gap is not formed, on the lower substrate; wherein: (i) when the air gap forming film is made of SiC, Si, or a group 3-5 compound semiconductor, the forming of the buffer layer includes sequentially stacking an oxide layer made of a metal oxide containing α-Al 2 O 3 or γ-Al 2 O 3, MgO, SrTiO 3, or YSZ, and a metal layer made of a platinum group metal containing iridium (Ir), platinum (Pt), or rhodium (Rh) or a material containing Ni; or, (ii) when the air gap forming film is made of SiC, Si, or a group 3-5 compound semiconductor, the forming of the buffer layer includes forming a layer containing SiC; or, (iii) when the air gap forming film is made of an Al 2 O 3, MgO, YSZ, iridium, quartz, platinum, or SrTiO 3, the forming of the buffer layer includes forming a metal layer made of a platinum group metal containing iridium (Ir), platinum (Pt), or rhodium (Rh) or a material containing Ni. (i) when the air gap forming film is made of SiC, Si, or a group 3-5 compound semiconductor, the forming of the buffer layer includes sequentially stacking an oxide layer made of a metal oxide containing α-Al 2 O 3 or γ-Al 2 O 3, MgO, SrTiO 3, or YSZ, and a metal layer made of a platinum group metal containing iridium (Ir), platinum (Pt), or rhodium (Rh) or a material containing Ni; or, (ii) when the air gap forming film is made of SiC, Si, or a group 3-5 compound semiconductor, the forming of the buffer layer includes forming a layer containing SiC; or, (iii) when the air gap forming film is made of an Al 2 O 3, MgO, YSZ, iridium, quartz, platinum, or SrTiO 3, the forming of the buffer layer includes forming a metal layer made of a platinum group metal containing iridium (Ir), platinum (Pt), or rhodium (Rh) or a material containing Ni.