An image acquisition method includes storing a coefficient of a relational expression between a parameter corresponding to a light quantity incident on an imaging sensor including a photo sensor element and an output value of the imaging sensor in the case of the light incident on the imaging sensor which employs a reference image accumulation time, inputting a desired image accumulation time, and calculating a parameter for obtaining a desired output value of the imaging sensor by using a corrected relational expression obtained by correcting using an output value of the imaging sensor employing the desired image accumulation time in the case of the incident light quantity being zero, adjusting the light quantity incident on the imaging sensor to be a calculated parameter, and acquiring a target image by the imaging sensor on which an adjusted light quantity is incident, and outputting data of the acquired image.