problems with array

PostgreSQL is 7.4.7.

My first table
CREATE TABLE tb_cat (
id INTEGER,
desc text
);
INSERT INTO tb_cat VALUES (10, 'cat10');
INSERT INTO tb_cat VALUES (20, 'cat20');
INSERT INTO tb_cat VALUES (30, 'cat30');

My second table
CREATE TABLE tb_array(
id INTEGER,
cat INTEGER[]
);
INSERT INTO tb_array VALUES(1, ARRAY [10, 20]);

When I write my select
SELECT * from tb_cat WHERE id IN (SELECT cat FROM tb_array WHERE id=1);
the output is:
ERROR:  operator does not exist: integer = integer[]
HINT:  No operator matches the given name and argument type(s). You may need 
to add explicit type casts.

Can anyone help me?
thanks.