"A UK exit from the EU would reverse the trend towards greater global trade and investment, and the jobs they create, and is a further serious risk to growth," the declaration says.
At the Japan Summit the G7 leaders are discussing the state of global economy and the migration crisis. The participants are also addressing cyber security issues and counterterrorism.
Read alsoJapan PM Abe warns of large scale economic crisis - mediaThe declaration "recognizes the ongoing large scale movements of migrants and refugees as a global challenge which requires a global response, in full respect for human rights and in accordance with applicable international law."
G7 states are the UK, Canada, France, Germany, Italy, Japan, and the United States.