A drive control mechanism and a safety device in a sewing machine in which thread in a predetermined amount necessary for the formation of one seam each time a seam is to be formed. The drive control mechanism comprises a control circuit for allowing current to flow to a machine motor when no thread amount signal is produced though a position signal is produced at the start of the machine. The safety device comprises a thread paying-out detection circuit operable when a position signal is produced. but no thread amount signal is produced, an alarm for producing a sound or light warning in response to the operation of the thread paying-out detection circuit and a motor stop circuit for inturrupting flow of current to a machine motor.