PURPOSE: To prevent the inconvenience which is given to the switching of mode due to an inadvertent operation and at the same time to simplify the mechanism, by preventing the action of a mode selection member when the electromagnetic release mode is changed to the machine release mode.

CONSTITUTION: The ordinary dial setting (1sec, 1/2sec...) is carried out in a electromagnetic release mode, and a short stroke is set for a release rod 15 by a clock lever 6. When the elecromagnetic release mode is switched to the machine release mode (M,B), the engagement between the contact part 6a of the lever 6 and a dial stage part 2a must be released. For this purpose, a release button 22 is pushed to give a right turn to a release lever 4. In such constitution, an inadvertent change of mode can be avoided.