A laser scanning microscope including a light source apparatus, wherein the light source apparatus comprises: laser diode elements adapted to emit laser light according to an inputted current signal, a light receiving element adapted to receive the laser light emitted from the laser diode elements, and a controller adapted to control light emission of the laser diode elements, wherein the controller includes a first laser diode element drive circuit that outputs a current signal to the laser diode elements according to an instruction signal, a second laser diode element drive circuit adapted to adjust the current signal on the basis of light quantity of the laser light received by the light receiving element, and to output the adjusted current signal to the laser diode elements, and a circuit switching section adapted to switch between the first laser diode element drive circuit and the second laser diode element drive circuit according to an instruction signal, wherein the circuit switching section is adapted to select the first laser diode element drive circuit, when fast-modulated laser light is required, so that the current signal is outputted to the laser diode elements according to the instruction signal from the controller, and wherein the circuit switching section further is adapted to select the second laser diode element drive circuit, when highly stable laser light is required, so that a current signal fed back with the light quantity of the received laser light can be inputted into the laser diode elements, characterized by a current detection section adapted to detect a current signal value to be outputted to the laser diode elements when the second laser diode element drive circuit is selected, and a memory which is adapted to store offset and gain for finely adjusting a drive instruction signal being changed in response to a change of the observation mode, wherein in a high resolution mode for performing high resolution sample observation, the memory is adapted to store a relationship between the current signal fed back with the light quantity of the laser light and the current signal detected by the current detection section in the high resolution mode, and wherein, in a high speed mode for performing sample observation irradiated with a fast-modulated light, the light source apparatus is adapted to correct a drive instruction signal for giving a desired quantity of light to the controller based on the relationship between each of the current signals stored in the memory. laser diode elements adapted to emit laser light according to an inputted current signal, a light receiving element adapted to receive the laser light emitted from the laser diode elements, and a controller adapted to control light emission of the laser diode elements, wherein the controller includes a first laser diode element drive circuit that outputs a current signal to the laser diode elements according to an instruction signal, a second laser diode element drive circuit adapted to adjust the current signal on the basis of light quantity of the laser light received by the light receiving element, and to output the adjusted current signal to the laser diode elements, and a circuit switching section adapted to switch between the first laser diode element drive circuit and the second laser diode element drive circuit according to an instruction signal, wherein the circuit switching section is adapted to select the first laser diode element drive circuit, when fast-modulated laser light is required, so that the current signal is outputted to the laser diode elements according to the instruction signal from the controller, and wherein the circuit switching section further is adapted to select the second laser diode element drive circuit, when highly stable laser light is required, so that a current signal fed back with the light quantity of the received laser light can be inputted into the laser diode elements, characterized by a current detection section adapted to detect a current signal value to be outputted to the laser diode elements when the second laser diode element drive circuit is selected, and a memory which is adapted to store offset and gain for finely adjusting a drive instruction signal being changed in response to a change of the observation mode, wherein in a high resolution mode for performing high resolution sample observation, the memory is adapted to store a relationship between the current signal fed back with the light quantity of the laser light and the current signal detected by the current detection section in the high resolution mode, and wherein, in a high speed mode for performing sample observation irradiated with a fast-modulated light, the light source apparatus is adapted to correct a drive instruction signal for giving a desired quantity of light to the controller based on the relationship between each of the current signals stored in the memory.