A production method for producing a thermoplastic resin molded product by using: a plasticizing cylinder which has a high pressure kneading zone for bringing in contact and kneading a molten resin, obtained by plasticizing a thermoplastic resin, with a pressurized fluid, and a pressure reduction zone for discharging the gasified pressurized fluid from the molten resin brought in contact and kneaded with the pressurized fluid, the production method comprising the steps of: (a) plasticizing the thermoplastic resin to provide the molten resin; (b) bringing in contact and kneading the molten resin with the pressurized fluid; (c) lowering a pressure of the molten resin brought in contact and kneaded with the pressurized fluid to separate the gasified pressurized fluid from the molten resin; and (d) molding, into a desired shape, the molten resin from which the gasified pressurized fluid is separated, characterized by: shutting off communication between the high pressure kneading zone and the pressure reduction zone and performing step (b) in a state in which the communication between the high pressure kneading zone and the pressure reduction zone is shut off; controlling a pressure of the pressure reduction zone so that the pressure is not less than an atmospheric pressure and the pressure is not more than a maximum pressure of the high pressure kneading zone which is achieved when bringing in contact and kneading the molten resin with the pressurized fluid, in the state in which the communication between the high pressure kneading zone and the pressure reduction zone is shut off; and communicating the high pressure kneading zone and the pressure reduction zone prior to step (c). a plasticizing cylinder which has a high pressure kneading zone for bringing in contact and kneading a molten resin, obtained by plasticizing a thermoplastic resin, with a pressurized fluid, and a pressure reduction zone for discharging the gasified pressurized fluid from the molten resin brought in contact and kneaded with the pressurized fluid, the production method comprising the steps of: (a) plasticizing the thermoplastic resin to provide the molten resin; (b) bringing in contact and kneading the molten resin with the pressurized fluid; (c) lowering a pressure of the molten resin brought in contact and kneaded with the pressurized fluid to separate the gasified pressurized fluid from the molten resin; and (d) molding, into a desired shape, the molten resin from which the gasified pressurized fluid is separated, characterized by: shutting off communication between the high pressure kneading zone and the pressure reduction zone and performing step (b) in a state in which the communication between the high pressure kneading zone and the pressure reduction zone is shut off; controlling a pressure of the pressure reduction zone so that the pressure is not less than an atmospheric pressure and the pressure is not more than a maximum pressure of the high pressure kneading zone which is achieved when bringing in contact and kneading the molten resin with the pressurized fluid, in the state in which the communication between the high pressure kneading zone and the pressure reduction zone is shut off; and communicating the high pressure kneading zone and the pressure reduction zone prior to step (c). (a) plasticizing the thermoplastic resin to provide the molten resin; (b) bringing in contact and kneading the molten resin with the pressurized fluid; (c) lowering a pressure of the molten resin brought in contact and kneaded with the pressurized fluid to separate the gasified pressurized fluid from the molten resin; and (d) molding, into a desired shape, the molten resin from which the gasified pressurized fluid is separated, characterized by: shutting off communication between the high pressure kneading zone and the pressure reduction zone and performing step (b) in a state in which the communication between the high pressure kneading zone and the pressure reduction zone is shut off; controlling a pressure of the pressure reduction zone so that the pressure is not less than an atmospheric pressure and the pressure is not more than a maximum pressure of the high pressure kneading zone which is achieved when bringing in contact and kneading the molten resin with the pressurized fluid, in the state in which the communication between the high pressure kneading zone and the pressure reduction zone is shut off; and communicating the high pressure kneading zone and the pressure reduction zone prior to step (c). shutting off communication between the high pressure kneading zone and the pressure reduction zone and performing step (b) in a state in which the communication between the high pressure kneading zone and the pressure reduction zone is shut off; controlling a pressure of the pressure reduction zone so that the pressure is not less than an atmospheric pressure and the pressure is not more than a maximum pressure of the high pressure kneading zone which is achieved when bringing in contact and kneading the molten resin with the pressurized fluid, in the state in which the communication between the high pressure kneading zone and the pressure reduction zone is shut off; and communicating the high pressure kneading zone and the pressure reduction zone prior to step (c).