Driving device and image forming apparatus

A driving device to be used in an image forming apparatus, comprising:

• an input gear;
• a partly non-toothed gear having a toothed portion engageable with said input gear and having a non-toothed portion;
• a holding member capable of placing said partly non-toothed gear in a holding state in which the non-toothed portion opposes said input gear and said partly non-toothed gear is not engaged with said input gear and capable of placing said partly non-toothed gear in a released state in which the holding state is released;
• a cylindrical member rotatable by receiving a rotational force;
• a driving member for applying the rotational force to said cylindrical member;
• and
• an entering portion which is rotatable together with said partly non-toothed gear and which is capable of moving inside of said cylindrical member and capable of receiving the rotational force from said cylindrical member by a frictional force between said entering portion and said cylindrical member.