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 <form class="fin-form-select" action="<?php bloginfo('url'); ?>/" method="get">
                            <?php
                            $select = wp_dropdown_categories('taxonomy=area&name=area&echo=0&value_field=slug&class=form-control');

                            $select = preg_replace("#<select([^>]*)>#", "<select$1 onchange='return this.form.submit()'>", $select);
                            ?>

                            <div class="container-select-border-brown">
                                <?php
                                echo $select;
                                ?>
                            </div>

                            <div class="container-btn">
                                <input class="btn btn-block btn-brown icon-search" type="submit" name="submit" value="検索する" />
                            </div>

                        </form>