Tapes japonica lysozyme (TJL), which belongs to the invertebrate-type lysozyme family, has a unique dimer formation. The residues, which include catalytic residues (glutamate 18 and aspartate 30), at the dimer interface form electrostatic interactions. Our previous study suggested that increasing the NaCl concentration switched TJL from a dimer to monomer structure, which increased TJL activity. Therefore, conversion from the dimeric to the monomeric structure is crucial for the TJL activity. In the present study, to further understand the effect of NaCl on TJL dimer formation, we examined the protein concentration and pH dependence of TJL activity in the presence or absence of 500 mM NaCl. TJL activity was suppressed at the high protein concentration. And the optimum pH of TJL activity was decreased in the absence of NaCl. These dependencies confirm the presence of electrostatic interactions between molecules of TJL in the dimeric form in an aqueous solution.