PURPOSE: To retrieve a desired frame of a roll film accurately without any error by making a correction corresponding to the difference between the counted value of blip marks and an expected counted value or outputting an alarm signal if the counted value of blip marks is different from the counted value expected at a main frame side when the 2nd mark is detected.

CONSTITUTION: When a decision circuit L judges that the error occurrence frequency γ/β based upon the number γ of errors and the number β of marks is ≥1/10, the alarm O is generated to display device or film abnormality. When the result obtained by the decision device L is ≤1/10, a decimal value X is fractionized by a fractionizing device P so as to correct the counted value α of blip marks 2 by the value of the decimal part X of the quotient. When the decimal value X less than 0.3, a computing element Q performs arithmetic α= N.Y, and when X>0.7, a computing element performs arithmetic α=N.(Y+1). When 0.3≤X≤0.7, it is judged that which of the computing element Q and R should perform arithmetic can not be decided on, and an alarm S is generated. Thus when arithmetic for corrections is performed, a mark detection command signal C is outputted preferentially to the output of a counter F as the outputs of the computing elements Q and R, and the counted values α and β of counters F and J are corrected.