A mean-field Landau free energy is formulated for a compressible diblock copolymer melt that microphase separates upon heating. Finite compressibility, which induces microphase separation in such a copolymer, is incorporated into the free energy through interaction fields for describing effective interactions between constituent polymers. The condition of microphase separation transition and equilibrium microphase morphologies are determined for the diblock copolymer of deuterated polystyrene and poly(vinyl methyl ether) as a model system exhibiting the thermally induced microphase separation. The Landau analysis reveals that microphase separation transition for the given copolymer is of first order in the entire region of composition. The phase diagram for the copolymer including classical microphase morphologies is shown to be apparently different from that for typical diblock copolymers exhibiting microphase separation upon cooling. In addition, the fluctuations of free volume are analyzed after microphase separation. Excess free volume is shown to be present in the domains of a more compressible component.