A printer comprising: an image data input unit (75; 505, 575, 576, 578, 579) that inputs image data; a line type light exposure head that is driven based on the input image data; a film transport device including a capstan roller and a pinch roller that transport an instant film having a pod portion containing a developing treatment liquid in a direction orthogonal to a longitudinal direction of the light exposure head, a pair of spread rollers that are arranged downstream of the capstan roller and the pinch roller in a transport direction of the instant film and spread the developing treatment liquid of the pod portion by squeezing the pod portion of the instant film and forwarding the instant film with pressure, and a power transmission mechanism that transmits rotational driving power of an electric motor to each of the capstan roller and the spread rollers; a rotary encoder that outputs an encoder signal including a pulse signal having a cycle corresponding to a rotational speed of the capstan roller; a pulse signal detection unit that detects the pulse signal from the encoder signal output from the rotary encoder; a control unit that controls the light exposure head based on the image data input from the image data input unit, includes a speed detection unit detecting a transport speed of the instant film based on the pulse signal detected by the pulse signal detection unit, controls a light exposure timing of the light exposure head for a line image corresponding to the image data in synchronization with the pulse signal detected by the pulse signal detection unit, and sets the number of the line images per unit forwarding length of the instant film to be constant regardless of the transport speed of the instant film; and a density correction unit that corrects an amount of light emission of the light exposure head based on the transport speed of the instant film detected by the speed detection unit and corrects a change in density of the line image caused by a change in the transport speed of the instant film; characterized by an effective detection period setting unit that sets an effective detection period (Δt) for detecting a subsequent pulse signal each time the pulse signal detection unit detects the pulse signal; and a pulse signal generation unit that generates the pulse signal after the effective detection period (Δt) in a case where the pulse signal detection unit does not detect the pulse signal within the effective detection period. an image data input unit (75; 505, 575, 576, 578, 579) that inputs image data; a line type light exposure head that is driven based on the input image data; a film transport device including a capstan roller and a pinch roller that transport an instant film having a pod portion containing a developing treatment liquid in a direction orthogonal to a longitudinal direction of the light exposure head, a pair of spread rollers that are arranged downstream of the capstan roller and the pinch roller in a transport direction of the instant film and spread the developing treatment liquid of the pod portion by squeezing the pod portion of the instant film and forwarding the instant film with pressure, and a power transmission mechanism that transmits rotational driving power of an electric motor to each of the capstan roller and the spread rollers; a rotary encoder that outputs an encoder signal including a pulse signal having a cycle corresponding to a rotational speed of the capstan roller; a pulse signal detection unit that detects the pulse signal from the encoder signal output from the rotary encoder; a control unit that controls the light exposure head based on the image data input from the image data input unit, includes a speed detection unit detecting a transport speed of the instant film based on the pulse signal detected by the pulse signal detection unit, controls a light exposure timing of the light exposure head for a line image corresponding to the image data in synchronization with the pulse signal detected by the pulse signal detection unit, and sets the number of the line images per unit forwarding length of the instant film to be constant regardless of the transport speed of the instant film; and a density correction unit that corrects an amount of light emission of the light exposure head based on the transport speed of the instant film detected by the speed detection unit and corrects a change in density of the line image caused by a change in the transport speed of the instant film; characterized by an effective detection period setting unit that sets an effective detection period (Δt) for detecting a subsequent pulse signal each time the pulse signal detection unit detects the pulse signal; and a pulse signal generation unit that generates the pulse signal after the effective detection period (Δt) in a case where the pulse signal detection unit does not detect the pulse signal within the effective detection period.