D3.js v4 Mapping Tutorial : TopoJSON

<!DOCTYPE html>
<meta charset="utf-8">
<html>
<head>
<title>D3.js v4 Mapping Tutorial : TopoJSON</title>
</head>
<body>

//パスジェネレーター生成
var path = d3.geoPath().projection(projection);

//地図用のステージ(SVGタグ)を作成
var map = d3.select("body")
	.append("svg")
	.attr("width", 960)
	.attr("height", 500);

//地理データ読み込み
d3.json("gunma.topojson", drawMaps);

//topojsonをgeojsonに戻す
function convertGeoJSON(topo) {
    return topojson.feature(topo, topo.objects.gunma);
}

//地図を描画
function drawMaps(topo) {
	var geojson = convertGeoJSON(topo);
	
	map.append("svg:g")
		.attr("class", "gunma")
		.selectAll("path")
		.data(geojson.features)
		.enter()
		.append("svg:path")
		.attr("d", path)  //dataに投影法を適応
		.attr("fill-opacity", 0.5)
		.attr("fill", "green")
		.attr("stroke", "#222");    
}

</script>
</body>
</html>

https://cdnjs.cloudflare.com/ajax/libs/d3/4.3.0/d3.min.js

https://cdnjs.cloudflare.com/ajax/libs/topojson/3.0.0/topojson.min.js