Content-Type 文字セット

What Is the Content-Type Charset Parameter?

The Content-Type HTTP response header tells the browser two things: the media type of the response body (like text/html) and — optionally — the character encoding used to encode it. The encoding is specified via the charset parameter:

Content-Type: text/html; charset=UTF-8
Content-Type: text/plain; charset=ISO-8859-1
Content-Type: application/json; charset=UTF-8

Without this parameter, browsers must guess the encoding using heuristics, byte-order marks, or HTML meta tags — a process that can go wrong and produce garbled text (mojibake).

Why It Matters for Unicode

Unicode text is abstract code points. To transmit it over a network, you must encode those code points as bytes. UTF-8 is by far the most common encoding — it can represent every Unicode code point and is backwards-compatible with ASCII. If the server sends UTF-8 bytes but the browser interprets them as ISO-8859-1, multi-byte sequences will be misread.

Example: The string "café" encoded in UTF-8 is 63 61 66 C3 A9. If interpreted as ISO-8859-1: - C3 → Ã - A9 → © - Result displayed: café — classic mojibake.

Header vs. Meta Tag

For HTML, the encoding can be declared in two places:

<!-- HTML meta tag (in-document declaration) -->
<meta charset="UTF-8">
<!-- or legacy form: -->
<meta http-equiv="Content-Type" content="text/html; charset=UTF-8">

# HTTP header (sent by server)
Content-Type: text/html; charset=UTF-8

The HTTP header takes precedence over the meta tag when both are present. The meta tag is a fallback for situations where the HTTP header is absent (e.g., opening a local HTML file).

Setting the Charset in Django

# Django sets UTF-8 by default in settings
DEFAULT_CHARSET = "utf-8"

# Response header automatically becomes:
# Content-Type: text/html; charset=utf-8

# Custom response
from django.http import HttpResponse
response = HttpResponse("Hello, 世界", content_type="text/plain; charset=utf-8")

Setting the Charset in Other Environments

# Flask
from flask import Flask, Response
app = Flask(__name__)

@app.route("/")
def index():
    return Response("Hello, 世界", mimetype="text/plain; charset=utf-8")

// Node.js / Express
res.setHeader("Content-Type", "text/html; charset=utf-8");
res.send("<p>Hello, 世界</p>");

# Nginx — add charset to responses
charset utf-8;
charset_types text/html text/plain text/css application/javascript;

JSON and charset

RFC 7159 and RFC 8259 specify that JSON must be encoded in UTF-8, UTF-16, or UTF-32. In practice, application/json is almost always UTF-8, and the charset parameter is technically redundant but harmless:

Content-Type: application/json; charset=UTF-8

Modern HTTP APIs typically omit the charset for JSON since UTF-8 is assumed.

BOM (Byte Order Mark)

Some tools prepend a UTF-8 BOM (EF BB BF) to UTF-8 files. Browsers recognize this as a UTF-8 signal, but the BOM itself is an invisible character that can cause issues in JavaScript and JSON parsing. Prefer the charset header over relying on BOMs.

Quick Facts