now=new Date();

mo=now.getMonth()+1;

if (mo<2) {
  document.write('<STYLE type="text/css"><!--body {background-image: url(http://www.asahi-net.or.jp/~xn4m-endu/image/background01.gif);}--></style>')
} else if (mo<3) {
  document.write('<STYLE type="text/css"><!--body {background-image: url(http://www.asahi-net.or.jp/~xn4m-endu/image/background02.gif);}--></style>')
} else if (mo<4) {
  document.write('<STYLE type="text/css"><!--body {background-image: url(http://www.asahi-net.or.jp/~xn4m-endu/image/background03.gif);}--></style>')
} else if (mo<5) {
  document.write('<STYLE type="text/css"><!--body {background-image: url(http://www.asahi-net.or.jp/~xn4m-endu/image/background04.gif);}--></style>')
} else if (mo<6) {
  document.write('<STYLE type="text/css"><!--body {background-image: url(http://www.asahi-net.or.jp/~xn4m-endu/image/background05.gif);}--></style>')
} else if (mo<7) {
  document.write('<STYLE type="text/css"><!--body {background-image: url(http://www.asahi-net.or.jp/~xn4m-endu/image/background06.gif);}--></style>')
} else if (mo<8) {
  document.write('<STYLE type="text/css"><!--body {background-image: url(http://www.asahi-net.or.jp/~xn4m-endu/image/background07.gif);}--></style>')
} else if (mo<9) {
  document.write('<STYLE type="text/css"><!--body {background-image: url(http://www.asahi-net.or.jp/~xn4m-endu/image/background08.gif);}--></style>')
} else if (mo<10) {
  document.write('<STYLE type="text/css"><!--body {background-image: url(http://www.asahi-net.or.jp/~xn4m-endu/image/background09.gif);}--></style>')
} else if (mo<11) {
  document.write('<STYLE type="text/css"><!--body {background-image: url(http://www.asahi-net.or.jp/~xn4m-endu/image/background10.gif);}--></style>')
} else if (mo<12) {
  document.write('<STYLE type="text/css"><!--body {background-image: url(http://www.asahi-net.or.jp/~xn4m-endu/image/background11.gif);}--></style>')
} else {
  document.write('<STYLE type="text/css"><!--body {background-image: url(http://www.asahi-net.or.jp/~xn4m-endu/image/background12.gif);}--></style>')
}