outputs pulses u2018 decoder output 1 u2019 in fig1 to start current supply from constant current sources 1511 and 1512 in response to the clock signal and the start pulse shift registers in the first stage unit start to operate and sequentially output pulses u2018 sr output u2019 in fig1 represents a stage number of from 1 to the last stage number the pulses denoted as decoder output 1 are continuously outputted until the shift register at the last stage in the first stage unit stops operating this ensures operation of the shift registers in the first stage unit then the second stage unit starts to operate here constant current sources 1513 and 1514 start supplying a current earlier than decoder output 2 for output pulses from a shift register 1505 at the last stage in the first stage unit are inputted to the nor circuit 1503 in fig1 showing the operation timing of constant current sources in the respective units u2018 ls current source u2019 in fig1 represents a stage number of from 1 to the last stage number outputs from the shift registers in the unit of the previous stage are used to turn the constant current sources on during a part of a period 1602 instead of outputs from the decoder after the shift registers in the unit of the previous stage finish outputting the.