diff --git a/CHANGES b/CHANGES
index 5e9d28cb6..05696fa37 100644
--- a/CHANGES
+++ b/CHANGES
@@ -58,6 +58,7 @@ Deprecated
 
 * Support for evaluating Python 2 syntax is deprecated. This includes
   configuration files which should be converted to Python 3.
+* The arguments of ``EpubBuilder.build_mimetype()``
 * The ``encoding`` argument of ``autodoc.Documenter.get_doc()``,
   ``autodoc.DocstringSignatureMixin.get_doc()``,
   ``autodoc.DocstringSignatureMixin._find_signature()``, and
diff --git a/doc/extdev/index.rst b/doc/extdev/index.rst
index 0dcd9a787..de2cbc1a0 100644
--- a/doc/extdev/index.rst
+++ b/doc/extdev/index.rst
@@ -242,6 +242,11 @@ The following is a list of deprecated interfaces.
      - 4.0
      - N/A
 
+   * - arguments of ``EpubBuilder.build_mimetype()``
+     - 2.0
+     - 4.0
+     - N/A
+
    * - ``nodetype`` argument of
        ``sphinx.search.WordCollector.is_meta_keywords()``
      - 2.0
diff --git a/sphinx/builders/_epub_base.py b/sphinx/builders/_epub_base.py
index f07fef21d..f20084d23 100644
--- a/sphinx/builders/_epub_base.py
+++ b/sphinx/builders/_epub_base.py
@@ -10,6 +10,7 @@
 
 import os
 import re
+import warnings
 from collections import namedtuple
 from os import path
 from zipfile import ZIP_DEFLATED, ZIP_STORED, ZipFile
@@ -19,6 +20,7 @@ from docutils.utils import smartquotes
 
 from sphinx import addnodes
 from sphinx.builders.html import BuildInfo, StandaloneHTMLBuilder
+from sphinx.deprecation import RemovedInSphinx40Warning
 from sphinx.locale import __
 from sphinx.util import logging
 from sphinx.util import status_iterator
@@ -470,9 +472,15 @@ class EpubBuilder(StandaloneHTMLBuilder):
         addctx['doctype'] = self.doctype
         super().handle_page(pagename, addctx, templatename, outfilename, event_arg)
 
-    def build_mimetype(self, outdir, outname):
+    def build_mimetype(self, outdir=None, outname='mimetype'):
         # type: (str, str) -> None
         """Write the metainfo file mimetype."""
+        if outdir:
+            warnings.warn('The arguments of EpubBuilder.build_mimetype() is deprecated.',
+                          RemovedInSphinx40Warning, stacklevel=2)
+        else:
+            outdir = self.outdir
+
         logger.info(__('writing %s file...'), outname)
         copy_asset_file(path.join(self.template_dir, 'mimetype'),
                         path.join(outdir, outname))
diff --git a/sphinx/builders/epub3.py b/sphinx/builders/epub3.py
index 88f93e90f..a20009017 100644
--- a/sphinx/builders/epub3.py
+++ b/sphinx/builders/epub3.py
@@ -78,7 +78,7 @@ class Epub3Builder(_epub_base.EpubBuilder):
         """Create the metainfo files and finally the epub."""
         self.validate_config_value()
         self.get_toc()
-        self.build_mimetype(self.outdir, 'mimetype')
+        self.build_mimetype()
         self.build_container(self.outdir, 'META-INF/container.xml')
         self.build_content(self.outdir, 'content.opf')
         self.build_navigation_doc(self.outdir, 'nav.xhtml')