I've followed several related solution. But I'm still facing the problem.
I've applied the following code for my custom post query by taxonomy (category). But it's not working. I think my problem with 'terms'.

    $terms = get_terms('service_cat');
    $services_post = new WP_Query(array(
        'post_type'         => 'service',
        'posts_per_page'    => -1,
        'tax_query' => array(
            array(
                'taxonomy' => 'service_cat',
                'field' => 'cat1',
                'terms' => $terms
            )
        ),
    ));

I also tried by the following code

    $services_post = new WP_Query(array(
                'post_type'         => 'service',
                'posts_per_page'    => -1,
                'category_name'     => 'cat1' 
    ))