diff --git a/README.md b/README.md
index 84c9db465..7186bc41a 100644
--- a/README.md
+++ b/README.md
@@ -361,6 +361,10 @@ irb(main):002> a.first. # Completes Integer methods
 - `VISUAL`: Its value would be used to open files by the `edit` command.
 - `EDITOR`: Its value would be used to open files by the `edit` command if `VISUAL` is unset.
 - `IRBRC`: The file specified would be evaluated as IRB's rc-file.
+- `XDG_CONFIG_HOME`: If it is set and `IRBRC` is unset, the file `$XDG_CONFIG_HOME/irb/irbrc` would be evaluated as IRB's rc-file.
+- `RI_PAGER`: The command specified would be used as a pager.
+- `PAGER`: The command specified would be used as a pager if `RI_PAGER` is unset.
+- `IRB_LANG`, `LC_MESSAGES`, `LC_ALL`, `LANG`: The first of these that is set would be used as the locale value.
 
 ## Documentation
 
diff --git a/man/irb.1 b/man/irb.1
index 93ef9b8f6..90cf5d4ae 100644
--- a/man/irb.1
+++ b/man/irb.1
@@ -180,7 +180,7 @@ The default value is 16.
 .El
 .Pp
 .Sh ENVIRONMENT
-.Bl -tag -compact -width "XDG_CONFIG_HOME"
+.Bl -tag -compact -width "IRB_USE_AUTOCOMPLETE"
 .It Ev IRB_LANG
 The locale used for
 .Nm .
@@ -190,10 +190,43 @@ The path to the personal initialization file.
 .Pp
 .It Ev XDG_CONFIG_HOME
 .Nm
-respects XDG_CONFIG_HOME. If this is set, load
+respects XDG_CONFIG_HOME. If it is set and
+.Ev IRBRC
+is unset, load
 .Pa $XDG_CONFIG_HOME/irb/irbrc
 as a personal initialization file.
 .Pp
+.It Ev RI_PAGER
+The command specified would be used as a pager.
+.Pp
+.It Ev PAGER
+The command specified would be used as a pager if
+.Ev RI_PAGER
+is unset.
+.Pp
+.It Ev VISUAL
+Its value would be used to open files by the edit command.
+.Pp
+.It Ev EDITOR
+Its value would be used to open files by the edit command if
+.Ev VISUAL
+is unset.
+.Pp
+.It Ev NO_COLOR
+Assigning a value to it disables colorization.
+.Pp
+.It Ev IRB_USE_AUTOCOMPLETE
+Assigning
+.Sy false
+to it disables autocompletion.
+.Pp
+.It Ev IRB_COMPLETOR
+Autocompletion behavior. Allowed values are
+.Sy regexp
+or
+.Sy type
+.
+.Pp
 .El
 .Pp
 Also