As long as your employer allows time off as maternity leave (paid or not), and you have the shakai hoken health insurance deal (many companies make you pay this by default, and JETs, BoE ALTs and some dispatch ALT employees will find themselves paying (a lot) for this every month) the government will pay you around 2/3rds of your daily salary for a period of up to 98 days. The schedule seems fixed to starting 42 days before the birth, and ending 56 days after. Not overly generous, but better than nothing.
If your company does pay for your maternity leave (though why would they when the government will do it for them?), then the government will only grant you payment if your company pays less than 2/3 of your regular wage (in other words, they will make sure you get 2/3rds of your wage as a minimum).
You can read more about the small-print on the (now defunct, but still accurate) Social Insurance Agency website:
How to Claim Your Benefit
To claim your benefit, you need to file the “Application for Maternity Allowance” (SHUSSAN TEATEKIN SHIKYU SHINSEISHO) to the Social Insurance Office (including one in a Social Insurance Bureau). We need certification by your employer and comment by your doctor on the Application form. For your first application, you need to submit photocopy of evidence such as your “payroll book” (CHINGIN DAICHO) and “attendance book” (SHUKKINBO). The deadline to file the claim is determined for each benefit payable day. You need to file a claim within 2 years from the next day of the day for which your benefit is payable.