trigger mutating

i have made a simple trigger whcih is doing nothing but just updating the attribute whose value goes down uder a specified value. code is

create or replace trigger dec_rawmaterial
after update on raw_material
for each row
when ( new.quantity_present < 500 )
begin
update raw_material
set quantity_present = quantity_present + 1000
where type = :new.type;
end;
/


what i do is

update raw_material
set quantity_present = 200
where type = 'PLAYCITE';

and error is

set quantity_present = 200
*
ERROR at line 2:
ORA-04091: table SCOTT.RAW_MATERIAL is mutating, trigger/function may not see it
ORA-06512: at "SCOTT.DEC_RAWMATERIAL", line 2
ORA-04088: error during execution of trigger 'SCOTT.DEC_RAWMATERIAL'

You cannot update the table that is being updated in an Oracle trigger.

If the type column is a primary key, you can change your trigger to: