新規にターミナル（端末）を開きます。ubuntu系は以下を同時押し。CTRL+ALT+T
#16.04 /mint 18 の場合
以下のファイルに1行書き足す。
sudo gedit /etc/apt/sources.list
deb http://download.virtualbox.org/virtualbox/debian xenial contrib
●Terminal / bash
wget -q https://www.virtualbox.org/download/oracle_vbox_2016.asc -O- | sudo apt-key add - wget -q https://www.virtualbox.org/download/oracle_vbox.asc -O- | sudo apt-key add -
sudo apt-get update
sudo apt-get upgrade
sudo apt-get install virtualbox-5.1
sudo gedit /etc/apt/sources.list
deb http://download.virtualbox.org/virtualbox/debian xenial contrib
●Terminal / bash
wget -q https://www.virtualbox.org/download/oracle_vbox_2016.asc -O- | sudo apt-key add - wget -q https://www.virtualbox.org/download/oracle_vbox.asc -O- | sudo apt-key add -
sudo apt-get update
sudo apt-get upgrade
sudo apt-get install virtualbox-5.1