ukired

Thank you for @bcworkz !!
I get it.

function.php

//pre_get_posts
function pre_get_posts_custom($query) {
	if( is_admin() || ! $query->is_main_query() ){
	return;
}
/*=====category===== */
if ( $query->is_category() ) {
	$slug = $query->query_vars['category_name']; 
	$cat = get_category_by_slug($slug);
	$query->set( 'posts_per_page', '2');
	$query->set( 'cat', $cat_id );
	$query->set( 'orderby', 'meta_value');
	$query->set( 'meta_key', $_GET['key']);
	$query->set( 'meta_value', $_GET['value']);
	return;
	}
}
add_action( 'pre_get_posts', 'pre_get_posts_custom' );

category.php

<?php if(have_posts()) : while(have_posts()) : the_post(); ?>

<!--post list-->

<?php  endwhile; endif; ?>
<nav id="pager">
	<ul id="pager_ul">
		<?php $maxpage = $wp_query->max_num_pages;
			$current = $paged;
			if(!$current){$current = 1;}
		?>
		<?php if(!($maxpage==1)): ?>
			<li id="prev"><?php previous_posts_link('前へ'); ?></li>
		<?php for($i=1; $i <= $maxpage; $i++): ?>
		<?php if($i == $current): ?>
			<li class="current"><?php echo $i; ?></li>
		<?php else: ?>
			<li><a href="<?php echo get_pagenum_link($i); ?>"><?php echo $i; ?></a></li>
		<?php endif; ?>
		<?php endfor; ?>
			<li id="next"><?php next_posts_link('次へ'); ?></li>
		<?php endif; ?>
	</ul>
</nav><!-- / #pager -->

<?php  wp_reset_query(); ?>