The envelope also contained a letter threatening to kill Yamamoto, the Jiji Press reported.
Police said they have launched an investigation.
Last week, another envelope was sent to Yamamoto with a knife inside.
The lawmaker was recently reprimanded for handing a letter directly to Emperor Akihito during a party last month.
Yamamato, an antinuclear activist, said the letter urged the emperor to pay attention to problems caused by the nuclear crisis at the Fukushima atomic power station.