An optical scanner (10A, 10B, 10C) comprising: a light source configured to emit light; a light deflector configured to deflect the light emitted from the light source to scan a to-be-scanned surface (15, 20) in a first direction and a second direction; a photodetector disposed on or above the to-be-scanned surface (15, 20) and configured to detect incident light and output a signal; and a controller configured to control at least one of operation of the light source and operation of the light deflector based on the signal output from the photodetector, wherein the to-be-scanned surface (15, 20) includes an optical scanning range on which the light is scanned, the optical scanning range including at least one detection field in which the photodetector detects the light, and an image drawing area on which an image is drawn, the image drawing area including a first area neighboring the at least one detection field in the first direction and a second area neighboring the at least one detection field in the second direction, and wherein the to-be-scanned surface (15, 20) is a screen having a microlens array on which a plurality of microlenses are disposed, and the to-be-scanned surface (15, 20) includes a reflection unit at least at an edge of the optical scanning range on the screen in the first direction, the reflection unit reflects the light to the photodetector, characterized in that the reflection unit includes a plurality of reflection planes (23a, 23b, 24a, 24b) disposed at intervals approximately equal to an array pitch of the plurality of microlenses that together configure the microlens array, and the microlenses are arranged in accordance with the sinusoidal trajectory in the main scanning direction, wherein the plurality of microlenses in the microlens array are arranged such that sinusoidal trajectory of light scanning the to-be-scanned surface (15, 20) approximately matches a center of each one of the plurality of microlenses, wherein microlenses that correspond to the pixels of the scanning line of the first half of the go and return scanning and microlenses that correspond to the pixels of the scanning line of the second half of the go and return scanning are arranged alternately in the horizontal direction. a light source configured to emit light; a light deflector configured to deflect the light emitted from the light source to scan a to-be-scanned surface (15, 20) in a first direction and a second direction; a photodetector disposed on or above the to-be-scanned surface (15, 20) and configured to detect incident light and output a signal; and a controller configured to control at least one of operation of the light source and operation of the light deflector based on the signal output from the photodetector, wherein the to-be-scanned surface (15, 20) includes an optical scanning range on which the light is scanned, the optical scanning range including at least one detection field in which the photodetector detects the light, and an image drawing area on which an image is drawn, the image drawing area including a first area neighboring the at least one detection field in the first direction and a second area neighboring the at least one detection field in the second direction, and wherein the to-be-scanned surface (15, 20) is a screen having a microlens array on which a plurality of microlenses are disposed, and the to-be-scanned surface (15, 20) includes a reflection unit at least at an edge of the optical scanning range on the screen in the first direction, the reflection unit reflects the light to the photodetector, characterized in that the reflection unit includes a plurality of reflection planes (23a, 23b, 24a, 24b) disposed at intervals approximately equal to an array pitch of the plurality of microlenses that together configure the microlens array, and the microlenses are arranged in accordance with the sinusoidal trajectory in the main scanning direction, wherein the plurality of microlenses in the microlens array are arranged such that sinusoidal trajectory of light scanning the to-be-scanned surface (15, 20) approximately matches a center of each one of the plurality of microlenses, wherein microlenses that correspond to the pixels of the scanning line of the first half of the go and return scanning and microlenses that correspond to the pixels of the scanning line of the second half of the go and return scanning are arranged alternately in the horizontal direction.