Abstract: A mechanism for selectively switching two drive transmission systems by one driving source is provided and a rocking lever which can be rocked between a first drive transmission gear of a first drive transmission system and a first drive transmission gear of a second drive transmission system respectively located on both sides of a drive shaft is provided. A rocking gear located between both drive transmission gears and selectively engaged with both drive transmission gears for transmitting driving force from the driving source is attached to the rocking lever and a locking means mocks the rocking lever in a state that the locking means is engaged with either of the both drive transmission gears. The first drive transmission system and the second drive transmission system are connected to a drive unit normally and reversely rotated. At least one flat side is formed in the above locking means and a flat side which can come in contact with the flat side of the locking means is formed in a fitting part.