Problem of trigger

[SD007]

Hi,I am trying to create a trigger on emp(employee_id,last_name,department_id) table before insert.The condition is the entered employee_id must not exist
in employees(employee_id is primary key) table and department_id must exist
in departments(department_id is primary key) table.The code is:-

create or replace trigger insert_emp
before insert on emp
for each row
declare
cursor my_cursor
is
select e.employee_id,d.department_id from employees e,departments d
where e.department_id=d.department_id;
emp_rec my_cursor%rowtype;
begin
for emp_rec in my_cursor loop
if ((emp_rec.employee_id=:new.employee_id) or (emp_rec.department_id<>:new.department_id)) then
raise_application_error(-20505,
'MAKE SURE THAT EMPLOYEE_ID'||' '||:new.employee_id||' '||
'IS UNIQUE AND DEPARTMENT_ID'||' '||:new.department_id||' '||'EXISTS');
end if;
end loop;
end;

Now,the employee_id condition is working fine,but whenever i am entering a department_id which exist in department table,the trigger is showing the massage "MAKE SURE.......".For every data I am inserting,department_id
condition retricting me to enter the data into the table.