A safety device for a machine has a first machine part for carrying out a working movement towards a second machine part. It also has at least a first and a second optical barrier moving along with the first machine part during the working movement. The first optical barrier runs ahead of the first machine part by a first distance, and the second optical barrier is arranged at a second distance from the first optical barrier. A control unit is designed to stop the working movement of the first machine part when the first optical barrier is interrupted. A test unit allows to test whether the second optical barrier has been interrupted once the first machine part has been stopped. A blocking unit serves for blocking the working movement as a function of the test result from the test unit. Using this safety device it is possible to determine an overtravel of the first machine part in a simple and reliable manner. Furthermore, an optical sensor barrier can be used to detect the position for a switchover between a rapid motion and a creep motion of the machine without requiring additional position sensors.