An improved light measuring device for measuring flash light is provided, and includes a sensor for measuring the quantity of flash light to produce a flash measuring signal. A starting signal is produced upon a detection of the start of the flash emission to enable the sensor for measuring the quantity of flash light. The light measuring device is kept in an operative condition for a predetermined time period after it is manually turned on so that it is available to detect the start of any flash emission during that time period. A counter can be utilized to set the predetermined time period, and the counter can be reset in response to any light measuring operation.