function manage_posts_columns($columns) {
	$columns['client'] = "クライアント";
	$columns['dairi'] = "代理店";
        $columns['thumbnail'] = "サムネイル";
	return $columns;
}
function add_column($column_name, $post_id) {
	if( $column_name == 'client' ) {
	$kuma = get_post_meta($post_id, 'event_date', true);
	$t = get_term_by('term_id',$kuma,'works-client');
	echo $t->name;
	}

	if( $column_name == 'dairi' ) {
	$kuma = get_post_meta($post_id, 'dairi_dat', true);
	$te = get_term_by('term_id',$kuma,'works-client');
	echo $te->name;
	}

	if ( 'thumbnail' == $column_name) {
	echo get_the_post_thumbnail($post_id, array(50,50), 'thumbnail');
	}

}

add_filter( 'manage_posts_columns', 'manage_posts_columns' );
add_action( 'manage_posts_custom_column', 'add_column', 10, 2 );