function getDistance(lat1, lng1, lat2, lng2) {
var lat1 = (+lat1),
lng1 = (+lng1),
lat2 = (+lat2),
lng2 = (+lng2),
lat_average = Math.PI / 180 * ( lat1 + ((lat2 - lat1) / 2) ),
lat_difference = Math.PI / 180 * ( lat1 - lat2 ),
lon_difference = Math.PI / 180 * ( lng1 - lng2 ),
curvature_radius_tmp = 1 - 0.00669438 * Math.pow(Math.sin(lat_average), 2),
meridian_curvature_radius = 6335439.327 / Math.sqrt(Math.pow(curvature_radius_tmp, 3)),
prime_vertical_circle_curvature_radius = 6378137 / Math.sqrt(curvature_radius_tmp),
distance = 0,
distance_unit = "";
//２点間の距離をメートルで取得する（単位なし）
distance = Math.pow(meridian_curvature_radius * lat_difference, 2) + Math.pow(prime_vertical_circle_curvature_radius * Math.cos(lat_average) * lon_difference, 2);
distance = Math.sqrt(distance);
distance = Math.round(distance);
// ２点間の距離を単位ありで取得する（1000m以上は、kmで表示）
distance_unit = Math.round(distance);
if (distance_unit < 1000) {
distance_unit = distance_unit + "m";
} else {
distance_unit = Math.round(distance_unit / 100);
distance_unit = (distance_unit / 10) + "km";
}
return {
"distance": distance,
"distance_unit": distance_unit
};
}

// 原宿駅:  [緯度] 35.670646 [経度] 139.702592
// 渋谷駅: [緯度] 35.658871 [経度] 139.701238
var d = getDistance(35.670646, 139.702592, 35.658871, 139.701238);
d.distance             // 1312
d.distance_unit    // "1.3km"