A head-up display apparatus (7, 77, 10) comprising: a light source device having at least one semiconductor laser that emits laser beam; an optical deflection device to deflect the laser beam from the light source device to a scanned face to generate an image thereon and a light detector; a light quantity adjustment device including a member disposable on a light path of the laser beam between the semiconductor laser and the optical deflection device to adjust light quantity of the laser beam responsive to a light transmittance target value; and an emission light intensity adjustment unit to adjust emission light intensity of the semiconductor laser responsive to an emission light intensity target value; characterised in that the head-up display apparatus further comprises: a controller to set a first value as the emission light intensity target value and a light transmittance target value when the laser beam is deflected to the scanned face by the optical deflection device based on a target luminance of the image input by a user or driver; and to set a second value as the emission light intensity target value when the laser beam is deflected to the light detector by the optical deflection device based on the light transmittance target value and the detection sensitivity of the light detector, wherein the controller determines whether the input target luminance is equal to or larger than a threshold value, the threshold value being set based on the detection sensitivity of the light detector, and wherein the controller sets the second value to be greater than the first value when the target luminance is less than said threshold value. a light source device having at least one semiconductor laser that emits laser beam; an optical deflection device to deflect the laser beam from the light source device to a scanned face to generate an image thereon and a light detector; a light quantity adjustment device including a member disposable on a light path of the laser beam between the semiconductor laser and the optical deflection device to adjust light quantity of the laser beam responsive to a light transmittance target value; and an emission light intensity adjustment unit to adjust emission light intensity of the semiconductor laser responsive to an emission light intensity target value; characterised in that the head-up display apparatus further comprises: a controller to set a first value as the emission light intensity target value and a light transmittance target value when the laser beam is deflected to the scanned face by the optical deflection device based on a target luminance of the image input by a user or driver; and to set a second value as the emission light intensity target value when the laser beam is deflected to the light detector by the optical deflection device based on the light transmittance target value and the detection sensitivity of the light detector, wherein the controller determines whether the input target luminance is equal to or larger than a threshold value, the threshold value being set based on the detection sensitivity of the light detector, and wherein the controller sets the second value to be greater than the first value when the target luminance is less than said threshold value.