Critical thinking is important to judge and diagnose plausibility of given information and to generate new information which can improve the old one. Two experiments were conducted to investigate the effect of writing on critical reading of an expository text. Experiment 1 examined whether a writing task helped Japanese undergraduate students, who are said to be less active in an analytic reading, critically read a short experimental research report. In Experiment 1 the subjects read a report, and then they responded to leading questions as they freely reread the text. The subjects also spoke aloud what they were thinking. Half of the subjects wrote down their responses (Writing Group), and the other half responded only orally (Oral Group). Constructive critiques the subjects generated while they worked with the text were analyzed. The number of critiques was significantly higher for the writing group than the oral group. Experiment 2 examined the effect of the external storage function of writing on critical reading. The number of critiques was significantly higher for the writing group than an “invisible writing” group in which subjects could not see what they wrote and a reading group in which subjects internally did the critiquing task. The findings suggest that the external storage function of writing promotes judging and diagnosing plausibility of the given information, that is, critical thinking in reading.