Domestic shipments of mobile phones fell 18.3 percent in August from a year earlier to 3.26 million units, the Japan Electronics and Information Technology Industries Association said Thursday.
The drop resumes a downward trend that halted in July, when a 13-month-long streak of decline was snapped.
The figure includes car phones but excludes personal handy-phone system handsets.
Including PHS handsets, mobile phone shipments fell 20.5 percent to 3.33 million units, for the 15th straight monthly fall, the industry association said.
Shipments of PHS handsets alone plunged 64 percent to 73,000 units, extending their losing streak to 18 months.
The shipment data were based on figures supplied by 16 mobile phone makers and 15 PHS manufacturers.